i probably should have mentioned that the iron sights on the rimfire MP5s suck a lot. it uses the normal HK style sights except that it is what you'd expect on a $300 rimfire clone. maybe the best way to describe it is, they are probably on par with what you'd get if you bought an MP5 airsoft gun. actually, i think you can even use a lot of airsoft parts on the rimfire MP5s.
i can't speak to the reliability of the GSG. i'm sure it is like all other rimfire semi autos and is picky about the type of ammo. if reliability was your goal, then i suggest a 10/22. it's not that a 10/22 is more reliable per se, but everyone knows how to take them apart, fix them, and install upgrades so you won't be SOL if something breaks or you need help diagnosing a problem. sure, a 10/22 isn't as tacticool, but it does give you a lot of options and offers a lot of upgrade potential.
i also want a real MP5, but even the clones are stupidly expensive.
i still think a dedicated 22LR upper for an AR is a good choice. also the 22lr bolt conversion is a great alternative and they are getting a lot cheaper. i think 22plinkster did a review on the CMMG stuff