One thing not commonly realized about the OM-2S, OM-3, OM-4, OM-3Ti and OM-4Ti is that all metering is done within the mirror chamber itself. The focus-screen is not used at all for exposure determination. This means that you can change focus screens or allow bright light to enter the eyepiece without it affecting the exposure reading at all.
The OM-2S does not have multi-spot metering, but for speedy handheld use, I prefer it to the 3(Ti)/4(Ti) bodies because in manual exposure mode, it is a live spot meter. The 3(Ti)/4(Ti) is centerweight averaging until you press the spot button.
The original OM-4 and the OM-2S are battery hogs. Even when turned off, they have a slight bit of battery drain. In actual operation they aren't any worse than any other OM body, but it's the slow leakage that will irritate you. Even so, I only go through two sets of batteries a year--big scary deal.
The OM-2S is also regarded as the least reliable from an electronics perspective. This is a bit misleading, though. The OM-2S experienced a larger percentage of failures in the first couple of years of operation. Once you get past that early failure point, they are probably as reliable as any other OM body. I've had mine since January 1986 and it has over 100,000 pictures on it and it'll probably work until there is no more film.