I tried out a F1, LMII and SA450 today.
The LMII is a bit warmer but the F1 has more of a bite IMO. The SA450 was just too big compared to the other two.
I ALMOST walked out of the store with the LMII but the ability to throw the F1 in my gig bag was too much to pass up.
I went for "bleeding edge" and am now a proud owner of an F1.
First impression (and only after 15 minutes of playing around with the dials on this thing) is this.... holy crap how does something just over 3lbs and the size of a hardcover book soung so friggin' loud and so good?

It is amazing.
I have it on top of my 6U backbreaker (50lbs+) and I noticed in my comparison that the F1 sounds about as good as my Ampeg head through a BBE Sonic Maximizer and a SansAmp RBI!
Note: I have yet to rehearse or gig with this thing but in the 15 minutes I used it in my basement at relatively low volume, I am floored how good this thing sounds.
I bought this head as a backup to my heavy rig and I was planning to use it just for rehearsal. I seriously think my Ampeg might be in danger of becoming an eBay casualty IF it gigs as good as it sounds at lower volumes.
I will post more detailed impressions next week after I have a chance to put it throught the ringer.