Usually a transmuter is a buffer/debuffer specialist. It has a lot of SoS spells, some of the bests SoD (no immunity for you, thanks), and some of the most powerful buffs, like the "bite of" series of spells, and the polymorph series. Also, you can find a lot of utility spells in that school, like fly or disintegrate. You can do some BC with the utility spells, but usually that's not the use a transmuter see, except for one or two spells.
So, you have two/three ways to play a transmuter:
1) Play like a cleric. That's a gish. Use all your buffs to improve your own combat prowess. cast bite of the werebear, greater blink and the likes, polymorph into an hydra, and use wraithstrike to hit your enemies easily. Or any of the other classical uses for buffs.
2) Use the buffs in your group. Way more benefical for the group, as many buffs will be more effective on your fellow warrior than they are on you. This can turn to be somewhat boring, mind you. The main thing you'll do here will be re-casting the buffs on your friends.
3) be a SoS/SoD specialist. Flesh to Stone, Slow, and the likes.
Of course, as a focused specialist you can do both 2 and 3, making the combat enjoyable while cooperating with the party. Illusion here works wonders as a complement.
Well, I may be wrong, but that's my experience as a transmuter. Self-buffing usually works better on "real" gishes, but you may do it even if you are not one. In general, transmutation is weaker than conjuration in the low levels, but surpases it on the higher ones.
