Until recently I have always been a charcoal/wood guy who believed that you could never get good results with Gas. However many people on here cook very successfully on propane and my chicken test suggests that the difference, if any, is minimal - with chicken anyway
When I got my gas grill (not dissimilar to the one in your picture) I had to make a few mods in order to keep a constant temperature within the chamber. I had to block up all of the "accessory" holes that allow you to insert spit turners etc. and also had to make a blanking plate that covered much of the large ventilation slot that is at the back. Once I had done this I could get reasonable temperature control at low temperatures indirectly by using the outside burners. If you get one make sure that it has at least 3 burners - preferably 4. Even with most of the accessory vents covered I do find it susceptible to rapid temperature swings if it is windy.
You can certainly get smoke by using a smoke box or foil packet of wood chunks/chips that you need to place on/near of the burners. These produce smoke for a short period of time and will need topping up regularly during a long smoke. I am not aware of a way of using wood blocks in a gas grill to get smoke though - maybe others on here can give advice on that. The charcoal smoker on the other hand will happily work away at the same blocks of seasoned wood for several hours producing a constant supply of smoke.
I have changed my opinion now and agree with the guys on here that you can do some good smoking on gas however I think it is a different type of smoking. Not better or worse - just different. The chickens I cooked (1 hour ish) showed little difference in final taste however when I repeated the test later with ribs (6 hours) the difference was marked.
Edited by Wade - 2/4/14 at 11:11am