I had a pair of ATC active 150's (half again the volume of the large 100's). they were spectacular, but I bought them used unheard, unseen, and they were too large for my room (and my room ain't small....15.25 x 23 x 10). My room doubles as a HD-based 110" home theater, and the sheer size of the 150's was too much for the scale of the room.
ATC active speakers are absolutely audiophile-grade transducers. Their midrange is legendary and they produce microdetail, harmonics and imaging that are out of this world. If I hadn't found great passive speakers in the meantime I would have gone with ATC SCM100ASL's for my mains (and Active 50's for surrounds). One problem: finding large ATC actives used is a pain, but new they are huge$$, even taking into account they cover both amp and speaker budget. The advantages of actives have been discussed here, but are clearly the near-perfect amp-driver integration, the dedicated crossover-amp synergy, and the convenience factors afforded by lack of speaker cables, etc. The results are quite evident; the linearity and "wholeness" of sound is astounding.
I HIGHLY recommend anyone searching for active speakers to go listen to ATC's big boys, the 50's on up. You will have an opinion, no doubt.