Problem was (last time I looked at them, admittedly _years_ ago), they have a fee to sign-up, and then there is a fee for almost every submission. Gets pricey. And then even if you're not passed up the line, you're supposed to get "detailed feedback." The people that I know that signed-up for Taxi were not happy with their success rate, not happy with their feedback, and definitely did not suck.
BTW, it's just a thing to be aware of... A point of terminology. It's said so often, regardless of how many times it's corrected, that sometimes we just let it slide by. BUT-- NEVER "sell" your songs. You are effectively leasing them, or leasing rights to exploit them, to a publisher. Do not sell your songs outright; it is an easy way to get ripped-off and lose all future earning potential for that material. No reputable publisher would offer a deal like that (outright sale). It's not how the business works. But just to be aware, you don't want to sell all rights to your songs, nor do you generally want to sign anything that has the phrase "work for hire," which effectively means the same thing. I don't really deal primarily in rap beats, but I've never understood the whole lease/buy/non-exclusive/exclusive thing in the hip-hop industry; it is so opposite to how the rest of the music publishing business works. And it has caused a few people problems (if you read some of the threads regarding "exclusive/non-exclusive"/beat-jacking issues here at FP).
In any event, don't "sell" your songs. That's how they got Charlie Parker back in the day, for like $5 worth of drug money...