Another UA refugee here. I used to blindly take UA on E145s and C7's or connecting through ORD rather than take a more convenient non-stop or more comfortable mainline aircraft. Now I take F9 for those non-stops from DEN that would normally be on a RJ with UA (BNA, CAK, etc) and I have been loving it.

As others have said:
+ Real elite boarding with no boarding scrum. I'm usually one of 2 or 3 Summit on a given flight.
+ Stretch is as good as E+. No FC, but that's ok. I have about 200 TED segments under my belt, so sitting in row 12 of an A320 is a natural fit.
+ Free - and good - beer.
+ Decent Buy on Board food.

What I really miss is building up UA or AA miles...but F9 comes in handy for getting tickets for relatives coming to town, vacation to Mexico, etc., and I'll save up UA miles for the big international trips.