My opinion is similar to the above.

The difference between C and F isn't much. The food is essentially the same, AVOD the same, etc. Its basically about the seat which is definitely better, but not astronomically so. After dozens of TPAC's in both C and F, I've made it my policy to not waste miles or cash on getting from C->F. If I can use a SWU, perfect, but its not worth cash or miles for me.

Then, last month, I was op-up'd on CX from C to F on a 6 hr flight within Asia on a 747. Even though it was officially operating as only a 2 class flight, the F/A's treated me like I was in F and the seat was SO much better. It highlighted how pitiful UA's F offering really is. The worst part is that I think the seats came from the same designer, but the CX one uses a bunch of the UA storage areas as seat space creating an extremely comfortable bed, while still having plenty of storage.

Long answer to a short question. UA F is nice, but to me, not nice enough to spend real money (cash or miles) vs C. Y vs C is a different story.