I've been using the Thunderkick, and I really like it. It feels surprisingly like playing a real bass drum, although the foam pads are maybe a bit soft. Mine eventually got compacted down to where the pedal was bottoming out, but I've been experimenting with other materials. I've found that stuffing a couple polyester socks in there works well and even feels more like a drum head to me than the original pads. The Thunderkick is a bit jumpy with a standard bass pedal. It's much more stable if the pedal has a base plate. Even so, I've had some issues with it creeping forward, so I usually wedge it up against a wall or other immovable object. The main reason I got it was for compactness and portability, and it's great for that. I keep a dedicated practice pedal attached to it and just move it around as one unit. When I add a lightweight hi-hat stand and a snare pad/stand, I have a "basic 3" practice kit that I can set up just about anywhere.
I have a pair of these Gibraltar type pads (not sure if that is the exact brand but it looks similar). They have some mass to them, are certainly more quiet compared to a drum, and have a bit of metalic timbre to them. The draw back I found is playing them on a plush carpeted surface as they flop around. I have used one of those orange photographic sandbags to steady it up.
I tried the ProLogix ThunderKick at NAMM a few years ago and really liked it, I dig the look of the product and found it appeared well contsructed too. I do like that idea that you hit 'into' the pedal...
However a short road test at a convention wasn't enough to get me to pull the trigger, but that was mostly due to have a rehearsal space for my kit and I really don't practice in the apartment anymore.
I think those would work better with my carpet situation, but it sounds like boomstick found some stability issues too, and it could probably use that same orange photographers sandbag to prevent creep.
I have the ProLogix Thunderkick attached to DW5000 w/a base plate. I love it for my living room practice pad kit - the portability is great though you probably want some carpet to work in conjunction with either velcro (on baseplate in my case) or spurs on the pedal. Otherwise, highly recommended - quiet and feels great to me.