It requires 8 ranks in skills, so level 6 entry at the minimum. So you get the awesome elemental at level 15, which is pretty late for effectiveness - at that point you can cast Greater Planar Binding as a straight wizard anyhow, and just bind an 18 HD elemental while keeping your spellcasting.

It's not the worst PrC or anything, but it's hardly that impressive. If your DM lets Extra Familiar from Dragon Magazine to qualify as an additional elemental, it could get much better though.

Interestingly enough, a Duskblade could qualify for this easily enough with the Obtain Familiar feat, and the immunities the class grants are pretty nice for a tank too. But the horrible BAB and HPs detract from that idea.