I bought the Icicle with Blue Microphones' Spark. It works really well: I plug in an XLR cable, and I plug in the USB output cable, and my iMac recognizes it, and everything works well! I'm really glad I bought it!
I bought a Blue Blue Bird Mic and this thing to use on my Mac. I was surprised at how well it worked after I adjusted the output level on the device. It's as lightweight as an empty cigar tube (almost like there's nothing inside of it) and feels a little cheezy, and the USB connection seems a little loose, but it works great. Simple plug and play. The sound quality is great.