The only negative feedback regarding to TST that I've for you is that not only you have to pay them 20% from your profits, but also you pay a lot of taxes since you are employed by TST which means you can't claim back any expenses like regular day traders do from trading their own accounts. Regarding to 80/20 split, I agree with Baron that it is a fair deal since TST provides you with their capital to trade.
The tax side sucks. You lose the 60/40 tax advantage from trading futures. You are a 1099 contractor yet the IRS definition of a contractor doesn't to me fit in with the rules and contract that they impose on you. Also by being a 1099 contractor you are subject to an additional 15.3% SE tax instead of 0 % that you would be liable from trading futures in your own account. The positive of being a self employed 1099 contractor is you can write off some of your expenses but depending on your profitability you are skirting a grey area as to whether your trading is hobby or business.
They serve a niche market and cater to people who may have talent but lack the means to act on it. One benefit I could see them providing is perhaps as an edge is knowing in the beginning it it is not your money at stake until you become profitable.