I haven't really played much of FFIX, just enough to et to the first save point, but from what I tried, it seemed like a fun game. Should probably try to get into it again one of these days.
I did, however, get much further with FFVIII, which I enjoyed very much. I think the point of summoning in that game is not so much to cause uber damage to your opponents, which it does, but to strengthen its bond with the character it's assigned to. As Cloral says, with the proper junctions your normal attacks become much more powerful, so it's interesting experimenting with that.
Also: Triple Triad. Hell of a fun game. There's a couple of fan-made replicas, notably this one. Be warned, you will become addicted.