If you want to find more information on how the 2009 Felt F4SL rides, you could also look at reviews of the 2009 Felt F3sl, 2010 F5, 2010 F4. They are all the same frame, the 2010 F5 is a slightly downgraded fork.

Felt's sizing in 54 and up is fairly standard, just a little low in front. There 52 and 50 are however, different. Essentially they have the same "reach" as the 54, just a lower headtube; but the 52 and 50 are virtually identical in both stack and reach. That is why for 2011 Felt is making a 51 and getting rid of the 50 and 52; and they are slackening the STA by a few degrees but leaving top tube length the same.

I can't really help you on comparing to the nanolight, but thats my take on the Felt. I'm hoping the Felt is being offered to you at a good deal because it's got the "older" 6601 Ultegra on it, and Felt is revamping their line for 2011.