NOTE: Every one of these establishments is worth visiting for the fish alone, so most of my comments focus on the atmosphere and other such things. As sushi tends to be, all of these restaurants are expensive. I would expect to pay over $30 for some filling combination of nigiri and rolls at Zip Fusion and Boss, and over $60 at Katana, Sushi Sushi, and Matsuhisa.
Katana - West Hollywood
The main thing that I liked about this restaurant was that there were approximately two cute, blonde hostesses for every old, Japanese sushi chef. There were over a dozen old, Japanese sushi chefs. It is located directly on the Strip, and, as would be expected, it is a rather trendy establishment. It seems like this would be a good place to go if you have a large group of friends.
Sushi Sushi - Beverly Hills
This is one of those traditionalist sushi restaurants where they will scoff at you with unspeakable contempt if you so much as think of ordering a California roll. Also, it was made clear on the menu that cell phone use would not be tolerated. Basically, they're serious. I remember that while I was there I got the brilliant idea to "rise through the ranks of tuna," which was awesome. The sushi is sold by the piece and otoro was $7. It is a very small and intimate place, there were only 2 chefs if I remember correctly, but it was not especially romantic, so this would probably be a good place to go alone (!) or with a friend that is serious about his sushi.
Zip Fusion - Downtown
This place is a gem in a not particularly nice area of the city. The restaurant is very colorfully decorated and the attitude of the staff is in sync with that. I sat at the bar, and the chefs were attentive and upbeat. The sushi here is very creative; they have all sorts of wild rolls and the presentation is, again, colorful as well as beautiful. I have since learned that the outdoor patio is very nice and is the place to be. This would be a fun place to go for a date.
Boss Sushi - Beverly Hills
Located on La Cienega, it is tucked away in some dumpy strip mall, but inside it is a bastion of trendy minimalism. The sushi here is still creative, but it is a little bit less so than Zip Fusion. I actually didn't eat any sushi here, but rather had the teriyaki New York Strip, which was fantasic. The reason I did not eat any sushi was that they had run out of toro, which is what I enjoy for desert and use to determine the quality of fish for comparison purposes. I think that was at around 7 PM on a Sunday night... Oh well.
Matsuhisa - Beverly Hills
I really didn't like this restaurant much, except for the fish which was, of course, excellent. It wasn't an especially appealing atmosphere, and the chefs weren't especially animated. None of these extraneous factors really worked against the place, but they certainly didn't help. Go for the sushi and sit at the bar.