dude i have the zoom ps-04 and its awesome! as far i know its very similar to the MRS series but smaller with less tracks....
drum machine is class and you can have a simple beat going in seconds. if you want to go that bit further though you can program the drums completely which is what im just starting to delve into at the moment. they're realistic as well- sound excellent. guitar sounds are ok stock but you can fully customise them anyway you want and get some pretty decent sounds out of em. alternatively, if your using a small practice tranny amp then you can run the guitar into the amp then connect a cable from the amps extension speaker jack to the zoom and get some really cool tones that way which i personally think sound excellent for certain things. all in all, its excellent for the money in my opinion. not sure on the exact differences between PS series and MRS but i know its not much. if the memory is anything similar though you will DEFINATELY need a bigger card- i would send you a recording of my band with the zoom but we havent managed to complete one yet as we keep running out of space!
need anymore info then ill be happy to help if i can, this is probably one of the few topics on here i can actually help a lot with.
this is an excellent forum for the zoom units, not too disimilar to here actually :)
http://www.2090.org/zoom/bbs/index.phptheres clips posted regularly and loads of info and help available.
P.S, as well as a drum machine theres a bass machine as well!!!