MAP sensor went after three weeks. Similar models struggle with the same issue. Once heard that heavily oiled air intake filters (K&N...) are not a positive thing to keep air intake sensors healthy - which was the case when inspecting mine.
Replaced rear shocks, brake pads, battery and some other basic stuff during the first months of ownership.
The car produces all kinds of rattles when warm/hot weather. The colder the weather, the more suspension seems to tighten itself up!
Rear beam rattle is most frequent/annoying.
The 306 gti is fast... but not lightning fast (or as fast as some tend to say). The engine only starts revving hard over 4000 rpm. Under that rpm, it feels a little underpowered. On the other hand, the fact it reaches 7200 rpm is a good thing: there's a very large power band. You need open roads to spread its legs. If you stay in the upper regions of the engine speed, it's OK.
Road holding is good, the car behaves in a stable way. Due to the construction of the rear beam, the car passive steers you through all kind of changes of direction with great ease. You can drive the car in a cool way, also at higher speeds, as long as you don't look for its limits. There's a great chance you'll find yourself with your nose in the wrong direction: over steer kicks in promptly, staying on the throttle in bends is an issue, progressive reacting a must. In not-dry conditions you need to calculate some margin when diving into corners. It's the kind of old fashion road manners you have to love and (be able to) control if you wanna give it a go.
Build quality is very good, as long as we're not mentioning suspension noises. Overall the car ages in a decent way.
Gear box is good, needs a little warm up to function at its peak.
Fuel consumption can be an issue: my average is 10-12 l./100km. (23-29mpg). I'm making lots of short trips, +30 mpg is not a problem when doing longer trips. I guess my right foot and an enthusiast driving style is the main problem.
For the money, it's still a good thing to buy. I like it's discrete way of being. Overall, it does what it's made for: going fast. Regular servicing is a must. It' not the cheapest way of driving. It should have been a little faster and a little easier to drive at the limit.