A bit over-hyped, but a good purchase.
First of all, Sweetwater is superior in customer service and tech support to just about any audio retailer I've experienced, so thank you guys.
I'm about a week into using the 18i20, upgrading from a 10-year old digi 002r. Definitely a step up and a good product for the price-point, but honestly it's a only a slight improvement.
Hardware: The preamps don't get real beefy up until about 3 o'clock, and suddenly you're clipping, and the knobs aren't very pro-feeling. However, the clarity of the preamps, S-to-N, and the A/D are better, and having two instrument DI's is a great addition. I must say the DI's are *very* warm, punchy and clear. Excellent for bass.
Software: MixControl has to be the most frustrating program I've ever had to use in my recording experience. It isn't intuitive in the slightest: you must RTFM, and when you seek help on focusrite's site, you'll find there's key info that's missing from the scarlett's page that's actually on saffire's. (A sweetwater rep had tell me that, much to our collective frustration.) If the 18i20 was, say, $200 more for the added benefit that MixControl was simpler, visually clearer, generally more user-friendly, and automatically stayed open in the background, I wouldn't hesitate to shell out the extra dough.
I do feel the overwhelmingly positive feedback on the 18i20 is a bit over-hyped and undeserved, mainly because of the software end, and focusrite's poor support. That said, for all the I/O and overall quality of the hardware relative to the very affordable price, it's a good buy.