Is FH1 fun? Oh hell yes -- it's the shit. It's definitely the most immense and complete WWII game out there PERIOD.
FH1 honestly made all my other WWII games at the time obsolete, and it always pissed me off that my PC was so slow I could barely play it. Often I would get less than 10 FPS, but I STILL would keep joining servers and trying to play. It was that good. I started playing FH1 back right before the 0.6 release I believe.
Ill admit, though, it is harder to go back now that FH2 exists. FH2 (regarding the same content it shares with FH1, of course) is far superior, in my opinion, than FH1 in every area except for air (and eventually sea) combat. That is only because of engine limitations in BF2, of course. The infantry combat in FH2 is much better than it ever was in FH1, which is the main reason I find it hard to go back.
Still, though, TOTALLY worth playing, if not for any other reason than to see where FH got its start. I'm not sure how often servers are full still, but I'm sure there are still some dedicated people that get on regularly. The sheer quantity (and quality) of historically accurate material in that mod is insane and will probably never be matched by FH2!