I bought an MZ-S + BG-10 grip cca. a year ago for about 200 GBP (and a second body for 165 GBP some weeks ago). I can say it's worth the money I paid. Small, versatile and (contraty it is packed with features) very easy to learn how to use... as usual with Pentax. Even if you just put a lens on it and switch it on you are ready to shoot.
AF does hunt in low light situations, but you cannot see properly anyway what you shoot at so it does not really matter. Not to mention that I personally rarely use AF in low light and if I use I use a flash as well.
The only big drawback might be that you cannot use FAJ type (aperture ring-less) lenses on MZ-S in every exposition modes.

Not that FAJ Pentax lenses were the league of lenses for this camera, but my Sigma EX DG 24-60mm is, and is a bit of disappointment that I can only use it in P and Tv modes. But I have learnt to live with the fact.

And yes: the plastic film door is a shame for a camera like this. Damn price cutting policy.
What compensates its minor weaknesses well is the fact, that you can use the Pentax MX's custom focusing screen (split image/microprism collar) with the MZ-S after a small adjustment. In a couple of minutes you can turn your MZ-S into a manualfocus top spec camera. I compared my two MZ-S bodies (one with the MX screen and one with its original AF screen) if there is a difference in measuring the light but I found no significant change. In fact they showed the same values with identical lenses fitted and apertures set. Kudos Pentax!