Aaron,
I think Taxi can be worth the cost - for the right person. As you likely know, there is an annual fee, plus a submission fee for each "song".
Often, the song is reviewed with suggestions, etc. and if you are serious about a song, you may re-submit it several times (each time for a fee and subject to review). If you send it in enough - and re-write it enough - and if the time is right (3rd Tuesday with a full moon) you may find a reviewer who will pass the song through to a publisher (I don't know if songs ever get direct to an artist rep - the stuff I've had luck with went to a publisher).
As you know, once at the publisher, it can sit in a catalog forever and not get recorded - or it may get recorded but there aren't enough sales to generate payments, etc. etc.
In my case, I gained access to a publisher who has worked with me (a foot in the door). I'm sure I would not have gained access to this publisher with unsolicitated mailings (maybe not even by camping out in the office
So for me, this contact was worth the investment. I've had a song placed in a film (direct to video - didn't pay much) and a couple of songs placed on artist recordings (no cash rolling in yet) - but I have an active and valid publisher.
As you know - Taxi looks for very formula driven material and you must be willing to write to that formula. In many cases (in particular placement for film) they look for well recorded material that can be placed as is (no recording costs).
I look at my Taxi fees the same way I do a gear purchase "will it pay for itself in x amount of time and will it improve my chance of making my demo studio profitable"
So far, I'm at a break even point with Taxi.