I've been researching this class, and it appears to be a more capable Warlock.  It uses the same basic mechanic as a warlock, and then it adds a ton of good stuff on top of that.

From : http://www.giantitp.com/forums/showthread.php?t=119814&page=2

Dragonfire Adept should not go below Tier 3.

Justification:

- Essentially a Con based class + d8 HD makes for a HUGE HP total. Probably higher than anything other than a raging barbarian. Even then, it might be equal as many barbarians focus on Str and Dex instead of Con.

- High AC: breath weapons have no ASF chance. Just stick the DFA in some full plate and you've got the AC of your standard tank.

- Crowd Control: Slow breath alone makes this class. This hurts melee'rs and casters alike.

- Damage: This only really takes off when you get to the Five Fold Breath of Tiamat, but you have 2 different AoE's and you can make your entire party immune to it. This makes it easy to use and extremely reliable.

- Utility: Great skill list. The ability to become quite good with these skills (an invocation that gives you a bonus to all knowledge skills and you count as trained in them all? Woohooo!). Let's not forget UMD as a class skill and all the other invocations that can give you important things like flight?

Many of his invocations count as buffs.....so he can keep them active all the time.

I'd say Tier 3.