Chances are good that you already know about the Remote AH, and how you can pay $2.99/month to buy, list, cancel auctions and so on. What you may not know is that the $2.99 is there if you plan on actually moving any product. If you just want to check to see what is listed on, say, the Neutral AH, that is free. Just use your smartphone or iPod Touch to download the Mobile Armory app and go to town.
You may be confused, as I was at first. The Remote AH is a part of the Mobile Armory, and it's ability to see currently listed items on the AH is a part of its free service. If you are like me, you have toons parked in the goblin cities to periodically check for deals on the Neutral AH, or perhaps even cross-faction toons to keep an eye out for arbitrage opportunities. Well, save yourself several log-ins and potential travel time by using the Remote AH (again, for free) to check up on those without ever having to log off your main character.
As for the actual monthly service, I do not think it is quite there yet. While it is fairly easy to search for specific items, you obviously do not have access to the addons you have when playing the game proper, some of which have more powerful searching and filtering tools (to say nothing about their listing powers). The one feature that would send everything over the top though, would be the ability to buy something on the Horde AH, list it on the Neutral AH, and then purchase it yourself for sale on the Alliance AH. Such arbitrage requires you to have a second account on hand to do it solo, but $15 extra a month is a bit steep in my opinion just to make some extra gold. $3 on the other hand? Instant sale.
If you are using the Remote AH and have some feedback to share, please do so in the comments.