We've had an MGF for a few months now and it's been great fun. Not 100% trouble free, but nothing major and considering it'd done almost zero miles over the past two years we were fully expecting a few niggles.
I think the main trouble with the MG is that "hairdressers" would like to drive one to pose in, but as a budget mid-engined sports car it's really more suitable for someone who likes driving and tinkering about with cars (i.e. not that "hairdresser" type generally).
Nearly all parts are really cheap, there's a pleasing lack of pointless electronic gadgets to fail, and it's surprisingly practical for that kind of car.
As for dropping the engine out to replace the spark plugs... why not use the big service hatch on top of the engine for that, like everyone else?
You do need to make sure the cooling system is in good nick and keep an eye on the coolant level (trivial to do since you can see the header tank every time you open the boot) - even if you do suffer head gasket failure you can have it fixed on your driveway by one of several reputable specialists for a few hundred quid. If you can buy a car with the gasket already replaced with the Land Rover version you're very unlikely to have problems.
In summary... not for everyone, but loads of character and great fun.