Also recently a lot of bloggers had their pagerank reduced for selling paid links through Text-Link-Ads, another good reason not to sign up with them.
Also TLA take too much of the revenue (50% for them and 50% for the blogger) which is why I now sell all of my sponsored links directly.
Another problem with TLA is that once you sell a link to a sponsor their price is fixed forever as long as they renew each month, even if your site gets really popular improves in Page Rank and attracts a lot more traffic. This is bad for the blogger and bad for TLA because if you’ selling a product/service which is consistently improving over time than it makes business sense to renegotiate prices every 6-12 months.