Type 4 (a.k.a. "pancake") engines have a nearly bullet proof bottom end as compaired to a Type 1 (a.k.a "upright" or "Beetle") style engine. The valve seats were always their weak point. From what I've always heard, the earlier type 4 engines (1972 was the first year) had less of an issue with the valve seats. A BIG advantage of the type 4 is they don't rely on a belt for cooling. The fan is directly attached to the crankshaft . . . so if the engine is running, the fan is turning. On a type 1, if you break the generator belt and keep driving, it'll overheat and melt down in real short order.
Some people swear by Type 4s. Some people swear at them. It's getting hard to find a good mechanic that will work on air-cooled eingines. Amoung them, like Nick was saying, it's hard to find one that will work on a type 4. Don't let that scare you though . . . they really are tough engines. Just more expensive to fix when something breaks.