I havent used one yet, but I can speak of the HD series, which is also full range switching. there is radio interference, but frankly, who listens to the radio anyhow with ipods, satellite radio, etc? Full range switching amps can sound very good. the HD series sounds fantastic, I would challenge you to hear a difference between them and the slash series (especially in a running car). I've also used some rather high end switching amps in homes and home theaters with excellent results. In fact, in my personal theater room, I run a 7 channel switching amp from Integra (I won't go into detail, but let's just say I have some pretty high end stuff in there, and for the money invested I could be driving something red from italy instead). It was not at all inexpensive, but sounds fantastic, runs cooler than my pre/pro, and has never tripped any breakers like my old amps would (when run really hard, which I have a tendency to do). Honestly, I believe switching amps are the future for everything. Unfurtunately, there has been a price barrior previously that made them not make sense (the switching devices need to be really high speed to sound good full range, not so much for bass duty). I think that may have been overcome now with the XD's, but I'll reserve final judgement until after I get to use one. I have eclipse switching amps in my 335 right now as a cheap stop gap until I decide between XD and HD amps, and they don't sound bad at all - pretty good, just not enough power for me... but they were pretty cheap and don't have any of the weird high frequency brittleness or fakeness I used to associate with most switching amps.
one important thing to note though, power output is going to be pretty much as stated - switching amps can't do ridiculous amounts more power after going beyond their capabilities - clipping is a different animal. On an AB amp, even after you run out of steam, there is often the ability to keep playing, just while clipping the signal (which is very bad) - without cutting completely out. On every switching amp I've heard, once you clip it, really really bad things happen - as it unloads completely, either cutting out, or making the speaker pop as it's power supply unloads into it. the point is, you need to know the limits of the amp and stay within them...
efficiency is paramount though, you can realistically get 30-50% more real world power in a typical car. easier on your electrical system, less heat generation, smaller - all good things. I do wish they made a more powerful version though, that's why I haven't just jumped right into the xd's and may end up with hd's. there's also the ZED leviathan, but I haven't heard to many good things. But I love the power output from that 6 channel. It's bigger than the xd, but would still fit in a couple of hidden places in the trunk