Greek Airport Project's Rhodes Diagoras International. When I first started flight simming, I set myself the mini-goal of recreating the flight from Manchester to Rhodes that I'd done as a passenger in real life a few years back. So while I was learning the basics and building up to the tubeliners, the first payware airports I bought were Manchester Xtreme from UK2000 and Rhodes Diagoras from GAP.
Rhodes was absolutely gorgeous when I loaded it up to test. I could recognise the hotel I'd stayed in and the old town looked fantastic. But when I finally got around to completing the flight, something terrible happened. Arriving round about the time the sun was setting, the framerate started to drop dramatically, until on final approach it was down to an unplayable <1fps.
It turned out that there's something in the night lighting or textures for GAP Rhodes that conflicts with FSX Acceleration. GAP say in their forums that they only support the product with SP2, have refused to consider any sort of fix and seem to grow quite irritated when people ask about it.
After wrestling with it for some time, even doing a complete reinstall of FSX to go back to SP2 for a while, I've now given up on it and installed the LiveInFSX version of the airport/island. It's maybe not quite as pretty as GAP's, but at least it doesn't turn into a pumpkin when the sun goes down.
The other purchase that I'm clooooooose to regretting is the upgrade from Just Flight's Traffic X to Traffic 360. While I can maybe accept that the greatly reduced volume of movements I'm seeing at my local airport (EGCC) may actually be more realistic than the taxiway queues I was seeing under Traffic X, I'm not convinced by the lack of variety of takeoffs : Leaving the sim running I clocked twelve takeoffs in about three hours, all but two of which were British Airways "Speedbird" call signs. The same time period in real life had takeoffs from many different airlines, at least according to Flight Aware. I've also seen AI flights in mid air with their doors opened and passenger stairs lowered. Traffic 360 may have a lot going for it, but there's definitely more than a few rough edges to it in dire need of a service pack.