I have to agree with Hand_of_Vecna. They're weak compared to a standard spellcaster, sure, but they're not BAD. I played a Truenamer once shortly after the book came out, and I didn't even know about Item Familiar. I don't remember what level we were playing at (probably 5 or 6), but I remember being able to get my Truespeak check high enough to use each ability at least 1/day without rolling, at least on creatures of my CR. I acted mostly as a support character by giving our tank extra attacks, freeing people from grapples, and of course, out-of-combat healing. The Identify utterance also came up a couple of times.
The Truenamer's utterances remain effective until about 9th level, at which point you seriously need boosters to get utterances off.
Really, a Legacy item with Item Familiar and the Amulet should be able to accomplish most DCs until you hit the high levels. The Item Familiar then becomes utterly vital.
The differences between Item Familiar and Custom Magic Items is simple: If you are using the latter, why are you playing a Truenamer?
