There is a general sense on the WoW forums - something I read regularly as a sort of barometer for AH behavior "in the wild" as it were - that the crafted JC rings and necklaces are terrible, a waste of time, "should be epic 359s" and so on. For the most part, such lamentations are besides the point. Whether something is objectively good or not has no bearing on its marketability, as any Mysterious Fortune Card or cigarette seller can tell you. These JC items being remarkably amazing would certainly help things along, of course, but it is not strictly necessary.
But are they actually any good?
The short answer: Sure.
The long answer: Ehh... some of them, yes.
When looking for advice or comparisons for items, Wowhead is the first place I go to. Not only is the website extremely elegant despite having a rather robust database of information, but it has a comment section for every item that is fairly well self-policed. Good posts generally get "greened" pretty quick and the garbage is thrown out. The problem when looking at the JC crafted item comments was that there weren't any. Until I made some. For all of them. My handle over on Wowhead is redraven937, and if you end up finding any of the comparisons useful, feel free to rate them up over there.
Instead of rewriting everything or simply copy/pasting what I wrote from Wowhead over to here, I'm going to do a quick summary. I still very much recommend checking out the items themselves to see how/why I arrived at my conclusions vis-a-vis whether the items are objectively good. Clicking on the item link will take you straight to my comparison comment. Keep in mind you can (probably) sell any of them at a 500g profit margin or higher regardless of how good or bad they are.
Elementium Moebius Band (tanking ring)
Verdict: Not recommended.
Reason: You get an extra +15 Stamina from this ring compared to the Revered Therazane ring, but most peoples' mains will hit Revered after questing in Deepholm + a few days worth of dailies, so at best it will be the Moebius and the Felsen. What I found was that I liked the ring from Revered with Ramkahen, the Red Rock Band, since it doubled up as both a tank and DPS ring. The Moebius ends up having 5-10 extra item levels over the Red Rock, but getting two tanking rings from two factions was too easy for me to recommend the pattern.
By the way, there are three JCs in the AH right now on my realm getting into an undercut war over these rings, which are the easiest the craft. Highest buyout right now is 2800g. There is still a margin in there, but it is looking pretty slim.
Elementium Guardian (tanking neck)
Verdict: Very Good
Reason: There is no reputation alternative for (plate) tanking, and this necklace stacks up pretty favorably against the necklace off the JP vendor. Easy way to save yourself 18 heroic boss kills and applying those JPs towards other upgrades.
Ring of Warring Elements (Intellect ring)
Verdict: Slightly Good
Reason: The crafted ring is technically better item-point-wise than the Therazane ring, but it comes down to Haste vs Mastery, with most spellcasters preferring the former over the latter. Using one of each is fine, but Lost City of Tol'vir has two chances of dropping sidegrades in a single (usually easy) run.
Eye of Many Deaths (Intellect neck)
Reason: This item is underbudget. Normally, a socket on an item of this level comes at the expense of 20 Intellect, but the Eye loses 40. Yes, the socket bonus grants 10 back, but that still puts it 10 Intellect down from where it should be. To get an idea of what this means, compared to the necklace off the JP vendor, the Eye has +112 Haste vs +20 Int, +92 Spirit, +3 Mastery - in other words, 112 vs 115, but 20 of those points are from Intellect (e.g. they are worth more) so it is even worse.
Two other issues are that A) Twilight Highland factions offer an epic caster necklace at Exalted, and B) the upcoming Love is in the Air holiday is most likely going to be dropping 100% sidegrades to this crafted necklace.
Elementium Destroyer's Ring (Agility Ring)
Reason: Simply put, this ring is probably overbudget. The other Agility ring is a joke compared to this one, as you end up looking at +30 Agility vs +27 Crit and +3 Hit - even if primary stats had the same item point budget as secondary stats (they don't), under no circumstances is that Agility gain not leaps and bounds better. The budgeting discrepancies continue when compared to the other 346 Agility rings, with the Destroyer consistently coming out 9 item points ahead despite being 346 like all the others. This ring is the closest you can get in WoW to a free lunch.
Band of Blades (Agility Ring)
Verdict: Mostly Garbage
Reason: Now we know where the Elementium Destroyer got all its extra lunch money... by stealing from its little brother. Essentially, Band of Blades does not compare well with the Destroyer nor does it compare very well with the Therazane reputation ring - it has 116 vs 107 from the Therazane ring, but 10 of that 107 is straight-up Agility. I am not aware of the exact formula of primary vs secondary, but when you have to bust out that particular design rubric just to figure out if a potentially 5000g ring comes up even with an easy reputation option, it is time to throw in the towel.
Obviously Hit rating can be important, which is the only reason why Band of Blades is "mostly" garbage. None of the rings are Unique-Equipped, so if you have a dearth of Hit rating on your otherwise amazing gear, you might get some benefit from dual-wielding (dual-knuckling?) these rings.
Brazen Elementium Medallion (Agility Neck)
Reason: Everyone who finishes questing in Hyjal will be Revered with the Guardians of Hyjal, which means all those people will have access to a necklace at level ~82 that they cannot wear until 85, but is basically identical to this crafted one. The Medallion (once socketed) will have +112 Crit over the Acorn's +96 Mastery and +5 Haste. Yes, 112 > 111. Incidentally, 5000g > 21g.
Update: As pointed out in the comments, Shadowpanther is actually showing this to be BiS for rogues, 2nd only to an epic 372 heroic Valiona neck drop. This obviously changes things quite a bit.
Entwined Elementium Choker (Agility Neck)
Reason: Against its Brazen brother, the Choker is down 11 item points. The Chocker is down 1 item point against the JP necklace. Finally, against the Acorn (aka Revered with Hyjal neck), the Choker sits at +65 Crit and +21 Haste vs Acorn's 86 Mastery, e.g. they're even. Individual stat weights will probably push the Choker ahead in that particular case, but nothing approaching it being more worthwhile to not just buy the damn Acorn.
Hopefully you will have found the above useful, as I spent a tremendous amount of time writing it all. As I mentioned before, whether or not the JC items are objectively good has no real bearing on your ability to squeeze some profit out of crafting them. I have personally sold two Eyes of Death for an average of 5500g thus far, and that is by far the worst/most underbudget option out of any of them. If all the rest were on the same level as the Elementium Guardian and Elementium Destroyer, we would probably see a lot nicer demand across the board.
But let's be honest here: we have all probably sold Ice Cold Milk, Enchanting Vellums, Vanishing Powder, and so on, at 1000% mark-ups. If we can sell items to people standing not even 20 yards away from the vendor we bought it from, we can sell rare BoE jewelry that looks/sounds cool for 4000g or more.