Well, so far I've made several fortresses that worked pretty well, in nice sports too(one had sand+magma for glassmaking fun). I do pretty well with the original 7, and can handle up to 25 dwarves pretty well. However, at this point my forts tend to stagnate, for several reasons:
1) I run out of ideas/things to do. When I get to this point, with a strong food production and craftdwarf's shops churning out trinkets, I end up thinking "now what?". I have a nice fortress with the supplies needed to fund a giant project, but... I don't have any giant projects to fun. The fortress WAS the project, and now it's done.
2) I can't find enough jobs for my dwarfs. With 15 or so skilled dwarfs, and couple haulers, a few militayr for guard here and there, I have all the basic needs met. When I get new immigriants, they do nothing but take up space and empty my food stockpiles(not that it matters since I'm making enough food to feed twice as many). I need something to do with these guys, so I'm actually looking forward to the next migrant wave instead of resenting it.
3) My FPS starts dropping. At this point, he game starts slowing down, which when combined with the above two and a lack of a clear goal, only makes me more likely to abandon this fortress and start a new one.
I've never actually faced any sieges/megabeasts, because I ever have the motivation to get that far with a game. The only Fun I've had is when I flood a fortress on purpose. I need some help on sticking with it and making something worthwhile enough to be invaded.