I have two actually (the new one just came in the mail after I sent that first comment lol) I have a 25W which is currently in a 2 gallon and the new 50W is in a 5 gallon. I've had the 25W one in the 2 gallon for the time I've had it (almost 2 months?) and it has worked flawlessly. I'm in the process of doing some tank rearranging and redecorating and it will be moving to a 3 gallon in the next couple of weeks I'm hoping. I'd like to try it out in a 5 gallon since it's rated up to 6 gallons, but my house is pretty cold (about 66-68F) so I go with 50W heaters for my 5 gallon tanks.
About the size, it's very very thin, and sticks close to the tank wall - but it's pretty long as far as heaters go, and can't be installed sideways. And that's the *only* con I can come up with right now lol