I was considering buying one of these pumps, then I had a friend overheat his system and kill his fish when he added one and I decided against it. My tank is in the garage which gets hot in the summer, the Velocity pumps suposedly add 3-4 degrees over a standard air cooled pump. I chose the Iwaki 40 RLXT, it is a little loud but it is a champ. If you are not concerned about temperature, I hear the Velocity is a great, quiet pump.
The CSL pumps are very quiet, but because they do add a lot of heat to a tank and get stuck periodically due to calcium deposits, I'm replacing my T2 with a Blueline HD40. One other nice thing about the CSL (well before CSL went defunct) is that the T pumps are pretty small.
T4's and the T3's will add heat to your tank. You should also consider getting a larger pump thats low RPM that will require less maintenance.
If you are going for quiet pumps there many now that are quiet and move a lot of water and impart a lot less heat.
The T3/4 require maint every 6 mo. i would run either vineger or diluted muriatic acid. Try to run them if you can while cleaning and not just soak them.
wow, i posted this over 2 years ago...i went with three velocity t4 pumps they have ran flawlessly with no heat problems the entire time. i have never taken them offline in over two years. this is one damn good pump