I have the chargriller duo, its thae gas grill on one side and then the smoker next to it. With the firebox and everything I thought it would be a sweet concept. Unfortunately piss poor craftsmanship and quality of materials make this a no no.
It seals like a screen door.
I have had to mod the heck out of mine to get anywhere close to a reasonable performance out of it.
I have another thread around here describing everything I have done, you should be able to search it down by my user name, but here is the short list.
1. Made charcoal box.
2. Mounted burner underneath to preheat the unit.
3. Mounted lowed rack and used flat chunks of soapstone to pull heat away from edge and create more even temps across the smoker.
4. Bought oven sealer rope and installed around edges to keep heat in.
5. Customized a water heater blanket and wrapped the hood.
6. Used a tall boy tecate can to drop the smoke stack down a bit inside.
This was all done either by learning from here or other resources, but it really was a pain in the ass.
As far as a grill goes, that thing should do quite okay for a cheap and will run up to 600 degrees in just a few minutes.
My recommendation would be to hold on to your cash for a while and try to find something better quality in the $1000 range. Buying a cheap model like I did only sets you up for dissapointing results in cooking, frustration to keep an even temp and burning through a lot more charcoal than you really should.