I own the behringer to800, which is a copy of the Ibanez tube screamer ts808. It sounds fantastic and the price was about an 1/8th of the Ibanez version; I got it new for $20.
The cheesy plastic cases tend to put people off, but under the hood the circuits are about 95% the same as the expensive versions with a few components tweaked to not infringe; it's up to yourself if that's ethical or not.
As for the plastic cases, the lack of shielding makes them prone to outside noise, but I sincerely doubt you'd be able to break one, as most online reviews tend to claim. They're made from the same type of plastic that motorcycle helmets are made from. Most behringers I've seen fail are due to issues with the switches they use or just abuse in other forms.
Overall, I think they're great for home use and noodling, especially since most of their lineup is tried and true circuits, but for gigging purposes I'd want a quieter pedal. I believe TC electronics just released a new series of pedals in metal cases that are just rehoused behringer circuits for a reasonable price.