I have a 300 watt running on my 55 gal
in line with one of my fluval 404 filters for 45 days. works great. Be aware that it must be verticle to prevent air trapping. Its kinda thick, at least twice as thick as a canisters 5/8 inch hose, so you need a little clearance behind the tank, or a stand that it could be under and still be verticle. I can hear it click on and off periodically.
Overall I am very satisfied. My tank is always 78 degrees, and I think the heat is distributed well since the heated water is pumped throughout the tank. And no heater stuck in the corner of the display tank! In my opinion, the only thing better would be a heater in a sump, so if you don't have a sump but do have a canister filter, get one!
Edit: Correction on size of in line heater - I was working behind my tank today and took a good look at my in line heater - its a little more than 3 inches wide and about 10 inches long - a bit thicker than I had remembered.