How does first-class aboard United Airlines compare to other airlines?

Was VERY fortunate to fly JFK to SFO on a United flight--seats reclined WAY back as there were only two or three rows. Configuration was 2--1--2, and they had wonderful treats such as rack of lamb and Godiva chocolates. Took a similiar flight on American, and I'd have to say United had a bit of an edge (little extra touches like a fresh carnation in the restroom). Both were nice, though, and wouldn't turn down either! I think a lot depends on whether or not the plane is a widebody ---there seems to be a lot more "perks" with a two-aisle plane, especially as far as seat comfort, as compared to the smaller ones with just one central aisle down the center of the plane.

I assume you mean domestic first class as that is totally different from international first class.

I don't fly first often, but I think that a great deal depends on what type of aircraft and where it is heading.
My wife and I flew first FRom Wash, DC to LAX and the plane was fantastic...but it was a plane that was continuing on to Australia..and had tremendous amount of room.

When we returned from our trip...we connected with a UAL flight in LAX and that plane had only 2 or 3 rows of first class...after our international flight it felt cramped.

Completely depends on length of flight and type of plane. Short routes (less than 3-4 hours) typically get short shrift and paying for 1st is hardly worth it. For longer, and particularly overseas, flights, most of the majors have very satisfying 1st accommodations/service/food. I'm referring to UAL/AA/TWA/Delta...don't know about NWA...Alaska's weak, Am West is weak). (Ever get a chance to fly Midwest Express, take it. All seats are 1st class. Prices are competitive. A true gem, especially from a value standpoint).
Overseas 1st class is truly night and day different from coach, esp if you're in coach on a full flight.
In answer to your question, UAL's 1st class is as good as any US carrier in my experience. Asian carriers are in a whole different league (better).

Another factor is whether or not the plane is a two-class or three-class plane. If it is a two class plane, the first class section is more like business class on the 3-class setup (although I prefer business class in a 3-class plane to first in a 2-class plane becuase you get the foot rest and the seats are more comfortable with more room.) If you are considering flying 1st class I think the previous posts are right on--it really depends on the length of the flight. Domestically, unless it is coast to coast, it's probably not worth the money or the miles if you upgrade. We just got back from flying from San Francisco to Munich via Washington D.C. in Business Class on UA's 777 and it was great (we used miles to upgrade). The service was wonderful, the food very good and the seats were great (much better than 1st class on a 757). With that said, UA's service is consistent, if not a little better than other domestic airlines but if you are flying overseas, the competition is better (Virgin, Air France, Singapore, Qantas, Britsh and Air New Zealand) all have wonderful business class services that are far superior to even UA's very good product. Hope this helps. Hans