Focusrite has relatively nice onboard preamps/converters... but lowest possible round-trip latency is not their forte'.
I have a Focusrite Scarlett 18i6 and it definitely doesn't beat out some of the competitors like MOTU or Roland (from what I've read anyway) when I set it to the lowest setting I'm getting around 7ms roundtrip which isn't noticeable at all for tracking (to my ears anyway). I probably wouldn't be able to get away with that with any less computer horsepower though and I have to increase the buffers slightly to avoid weirdness when using some of the heavier duty effects in Sonar.
The mic pres are very nice sounding and the unit hasn't given me any problems at all since I bought it. I could record a full band demo or properly mic up an entire drum kit with all the analog inputs if I want (need a mixer though). I can get even more inputs if I use a second box like a mic pre or my Echo Layla via the optical connectors.
But, as I said I have a pretty decent system. i7 2600k, 16GB RAM, 7200rpm HDDs, optimized for audio, etc...
Not really directing that at you, Jim as I know you know what's up. Just wanted to give some insight on the Focusrite stuff from a happy user and expand on your statement.
Essentially... yeah, not the fastest interface but with my system which is nice but not crazy it works great without any noticeable latency. I probably would have had to spend at least twice as much to get something in the ultra low latency realm that did as much as much and sounds as good as my Scarlett does.
Another advantage of a USB interface is you can plug it into other systems easily. Another reason I wanted a USB device. It lives on my big rig but I intend on using it with my laptop when I need to record somewhere other than my "studio".
I'm not a tech or pro engineer though so YMMV.