I tried the Focusrite 18i20, but handed it back.
First, the sound was not much better than my rather cheap Behringer FCA610 (ie the difference was subtle). Second, it had a rather annoying background hiss, which came out through the speakers. I think most sundcards produces some hiss, the FCA610 also, but the amount of hiss the Focusrite produced, was WAY higher.
Recently I bought an Esi Gigaport HD+. It does produce a little bit of hiss when gain are set to max, otherwise it does not bother me at all (Im very picky about that). It is at least a couple of steps better than FCA610 in sound quality. It does not sound harsh at all IMO.
The Gigaport HD+ supports only 6 channel 24 bit at 48 khz, - if you want to use all 8 channels, it will degrade to 16 bit. I havent tried using it with all 8 channels, so I do not know how it sound in that mode.
It has been on the market for quite some years, but it is, no contest, better than the two other soundcards I have tried. Also, the drivers are ROCK stable!!!
Even though it runs on only USB bus power, it has quite some output gain (which were one of my main concerns). It may not be enough in every system, but I find it perfectly fine for now.