My current project tried it. Spent $300 to join, then something like $15 each time we submitted a song. At the time, we were on a really small local label that had paid for our studio time, the first 2500 CDs and put us on the road for a week's worth of gigs, before they went totally bankrupt. Anyway, we were getting some airplay with two of the singles on the record and ended up selling out of the first 2500, just from online sales, selling at shows, and at local independent record stores.
Anyway, we figured we had a pretty good product so we were looking for ways to take the "next step." The reviews we got from TAXI almost seemed canned. Some were constructive, but most just seemed geared towards giving us just enough encouragement to try again... and again... and again...
I must say though, when I called them and explained that I didn't think their services where something we really wanted, they were quick to refund us our $300. In the end, it only cost us $45 to submit those three songs.
My feeling on it is that there must be a better way to get your stuff heard. Of course, if there was a silver bullet, everybody would just do that. There's still no substitute for playing live if you want to move merch. BTW, one of those singles was picked up by a publishing company that gave us an advance and is currently talking to FOX.... I'll just die if it ends up on the Dawson's Creek Reunnion or some crap like that...