I bought one, and it's not bad. I had it on a 10G Nano w/ a 10g sump and it was all it could do to keep up w/ the heat from my lights. I now have it on my 75G w/ 20G sump and it does well at maintaining the temp 1 degree on either side. The unit I have is the smaller of the two, rated for a 55G tank, and I think thier rating is WAY over, it probally is unable for a 20G or so.
When I first go the unit, I had a problem with the power supply. Drs. Foster & Smith took it back and had me another one in a decent ammount of time. Was is worth $300? Well, it kept my 10G going despite my 4 tubes, now? Well, since nothing comes close to the cost I'd still go for it. The bad boy chillers are just so expensive.
FYI, this unit is nothing more than:
2 Peltier junction coolers/heaters, 2 fans, and a thermostatic controller. I believe it has a SS liner in it, but could be mistaken. The unit is packed w/ a lot of styro so it's hard to disassemble w/o damaging something.
Just don't buy it and expect miricales!