I use flightmemory, but also openflights.org. Each has some features that are better than the other, so that's why I use two.

The thing that annoys me to no end about flightmemory is the map, and how you have to pick one domestic map (I have a lot of domestic US and domestic Canada segments, and can't see them at the same time). You also can't zoom, etc. The maps on openflights are much better, IMO. Plus, I like the fact you can download your map as a .kml and import into Google Earth, its pretty cool. You can also easily filter your flights in openflights, but not in Flightmemory.

However, flightmemory does have some advantages. Its easier to track other non-map stats, such as specific aircraft you've been in, and I like the fact that future flights aren't considered in your stats until you have taken them. I also like that fm will post to my twitter, and openflights to facebook.

What I would really like to see is one of these integrate with tripit, so that itineraries posted there would auto-populate.