The ES from Ashbys is good, even though mine seems to have some maintainance problems. I don't see how it could ruin any carpet, and even if the carpet is clean, you can work so much faster. On clean carpets I hardly use any chemicals anymore, the powerbrush with the steammate running does the trick fine.
I am a powerbrush addict now, and use it on almost every job - plus, if you have it, you can do the commercial work as well. Try cleaning 3000 square metres of dirty carpet with a wand!!