I think the Olymus OM series are the best SLR system ever made (Can you tell im biased? haha).
Previous posts have covered most points already, but i thought i'd mention my experiences with OM's anyway.
I have and use the OM1,OM2,OM2n,OM2SP, and receiving an OM3 very shortly. Cant wait.
My hands are on the larger size, but somehow the OM's feels very comfortable to use, actually everyone i've given them to report the same things. They are built extremely well. The light seals will die, but it's a very easy thing to replace. I do it whenever i get another OM and takes me about 15 mins now. To be picky, the only thing that could be better as far as build quality goes is the plastic timer lever on the front. Whilst this has never happened to me, i see others that have been broken off. Either they were forced, or caught on something that snapperd them off.
The VF is simplay amazing, metering brilliant, they're very user friendly, and until recently very cheap. I say "recently" because it looks like their average price has been increasing considerably over the last year or so.
It goes without saying how good Zuiko glass is. But there are some lenses that have become extremely over priced, such as the 40mm, which collectors have seemed to latch onto now.
As far as batteries go, thats a non issue with the OM1 and OM2/2n, but i've heard bad reports about the OM2SP. Personally, i've had the same batteries in it for a year now, so must not be expereinceing the problems others have reported.
My favourite every day camera to use is the OM2n. I like the aperture priority when im feeling lazy, or need a take a quick shot without having time to adjust shutter speeds etc. The OM1 is a masterpiece in its own right though. Perfectly balanced simplicity and reliability. I'll be receiving my OM3 any day now, i hope it compares as favourably as the others!