Disclaimer (lol, who puts a disclaimer in there messages): Somewhat long winded, but probably worth reading if you're intrested in the topic.
Although I'm not as good a mechanic as Edward, Im working as a mechanic for Ferrari, more of an apprentice at this point, when I'm not at school. So under a watchful eye (the head mechanic, he's the man, and since there are only three mechanics including me it is very cool) I've replaced clutches, cleaned a lot of parts, done major services, etc. on just about every ferrari from the 1970s on.
I too have a jeep wrangler and it is in fact very easy to work on. It's such a pain in the ass though to tune the damn thing. I've swapped it to a weber carb and a DUI distributor, and then had everything else apart to replace/fix as it has 253K miles on it now. The original clutch was just replaced 15k miles ago actually (I didn't do that), nor did I rebuild the engine when that was done. But every other part on the car I have taken apart. The Ferrari is definitely not the jeep. How many Ferraris get 200K miles without doing any real maintenance other than oil changes? The Ferrari is a whole hell of a lot more fun to work on though. Going down to work on the Fcar is fun, I may curse alot and occasionally catch myself preparing to throw a wrench at it, but it is fun. Working on the jeep is simply work to keep it running. So even though a Ferrari is a lot more work than a jeep (no matter what anyone tells you, it is a hell of a lot more) it is enjoyable.
Another random point of mine on maintenance: people will tell you a Ferrari is reliable, these people are 1. mechanics that own a Ferrari. 2. someone that has there car serviced every year. Ferraris are not reliable if treated as a normal, disposable car.
Now as an answer to the question. (Now remember through the rambling I said that the jeep is an easy car to work on and the 308 is hard compared to it, so everything here is Ferrari terms): The 308s are easy to work on. They are simple and parts are cheap (if you don't get them from Ferrari). 308s can be a royal pain in the ass to tune though, especially carbed cars (carbs and mechanical distributors - do any two actually advance the same?). All and all though, 308 is a good car, especially when you get to the later models where it avoids some of the older mechanical calibrations and switches to digiplex.
The 355 is the best made Ferrari ever. 9 out of 10 (maybe even 10 out of 10) mechanics will tell you that if they had a Ferrari, the 355 would be it. Ferrari just though of everything when they designed it and the thing is near bombproof. If you're mechanically inclined you can do a lot. It is somewhat tricky though and you have to be near a full blown mechanic to do everything. At my level now I could do nearly everything on a 355 so it's not impossible. The more modern cars are alot of it works or it doesnt work, i.e. the computer is working. So when it doesnt work you go down to Ferrari, bend over, and leave with a new part that all you have to do is install and tada it's working like there was never a problem. General maintenance can be done though in a household garage. When something is totally f_ed up though to Ferrari the car would have to go for diagnostic check and for someone that works on them 50-60 hours a week to look at.
The 550 is like the 355, a little tougher though to work on, in particular trans related things. Also, parts on the 550 are more expensive. The 550 isn't quite as good as the 355 imo regarding reliability, close though.
So, if I had a 355 and 550, I'd be rich. But if I was rich and bored, then I would work on them except for crazy things related to the computers, where you need the diagnostic stuff and a lot of spare parts to swap in and out, etc to see what's gone haywire. Another point here is that very little goes wrong with the newer cars. We get a fair amount of computer stuff, convertible tops leaking (the pistons, not the top itself), but this amounts for maybe 40% of the day to day work that comes in. The rest could be done at home if the person was a mechanic and had a household garage with a lot of tools. For someone who is not a mechanic, the 355 is kind of bordering on being a car to work on and not, the 550 most of the time is a little too "interesting" though to do at home though. It would depend entirely on the owner's ability, neither the 355 or 550 is a car to learn on though.