I think Skybird's assesment there is a fair one. LOMAC tends to prefer PCI-x graphics cards from what I can gather, and I've known people who simply cannot get it to run a server (which meant I had to do it), although I have run a server and played it at the same time with two other people on and most of the graphics up reasonably high, so it's not as though it's a slideshow if your PC is reasonably up to date.
LOMAC looks great from a 3D graphics standpoint, the planes look very convincing indeed and have cool virtual cockpits that work well, although the ground detail isn't up there with something like FS, for a combat sim, it's certainly among the better ones, the sim also implements stuff like heat haze on the exhaust plumes and was in fact one of the first sims to do that, so it's a looker, no doubt about that.
However, LOMAC is more of a 'survey sim' than a 'study sim' in that you won't find every switch working in the cockpit, and some aspects are greatly simplified in comparison to the real thing (notably the missile launch parameters and the radar operation). Since the radar and other sensors are very much the heart of a modern combat aircraft, you might find this ultimately disappointing, if that was where your interest was coming from, and if you were interested in the weapons side of things, and the cockpit controls to launch them, then Falcon AF is more likely to blow your skirt up.
Personally, I think LOMAC falls between two stools in this respect, being not complex enough to please the 'rivet counters', yet slightly too complex for the fun brigade, although the difficulty sliders can alleviate this a bit. It's sort of hard to place my finger on why what Skybird said about LOMAC being a bit sterile in feel is true, but it's what I feel about it too. The moment one fires up Falcon for example, it's a case of 'ooh yeah baby, now that's what I'm talking about...', similarly, stuff like wings Over Europe may be simplified for jets, but you feel they got the snips in the right places, and it also puts a smile on your face. I've never had that feeling from LOMAC.
What a lot of people were disappointed with by LOMAC (apart from it's legendary delayed release), was that it 'talked a big fight' promising to be 'the new Falcon 4' with superb realism (this was before Falcon AF came out), but when LOMAC came out, it became apparent that it was long on promises, and short on delivery.
Others may feel different of course, and if it floats their boat, then that's great, I'd suggest trying it out, it may be what you want, so judging it for yourself with the demo might be better than going off my opinion, which is after all, just an opinion.
Chock