whisper 20i filter any experience?

I've never used one before, but to me it basically looks like a power filter that fits in the tank instead of outside. Personally speaking I'd probably go with an AC 20, unless the tank is going to be right up against a wall. Then I'd look at internal solutions.

I have a 10i internal for my quarantine tank. If it were a permanent thing, something outside the tank would be better. This thing takes up room that the fish could be swimming in. However, I will say that it is very quiet. The hook on the back that hangs over the tank is large and too wobbly (much wider than my black frame molding). So that it doesn't rock around I had to put a piece of hard styrofoam to fill in the gap.

Edit... just saw your reply. It does have different adjustments on the hook to raise and lower it, so yes, if you place it working under the water surface there is less disturbance.

Even if your injecting CO2 I don't believe a filter such as an AC 20 would produce so much surface agitation that it would disipate much more than the internal whisper filter. If your not injecting CO2 I don't believe it would matter. Also people wouldn't have to stare at your filter when looking into the tank. In an ideal world you'd be able to try both before buying. Things being the way they are I guess it's pretty much just up to your judgement.

Buy a Whisper 20 hob filter. They take up about two inches from the tank to the wall. I grew plants in my tank with this filter on it with absoloutly no co2 or whatever. They grew like crazy. They do provide enough o2 for fish, though. The 20i is a great filter also. To save money I would buy one from Wal Mart instead of the LFS. I must say though, Whisper is the way to go!!!!!!