TW and I had a decent length discussion on these some time ago. Pros, cons, how well they truly work, etc.
They do work, but they have some shortcomings, some due to the design of the smokinator, some due to the limitations of the kettles; temp spikes, small water pan, can't do really long cooks are the biggies.
Since I didn't want to deal with refilling the water pan every two hours or thereabouts, I ended up just going and buying two firebricks and sticking them in a 22" OTS and foiling the firegrate that was under the food area to prevent drafts. You get a more constant temprature with the firebricks and it'll cost you all of about $5.
The biggest problem with using a Weber kettle for a smoker is air control. Unless you have one of the old triple rotating vent grills, the OTS & OTG grills don't seal well on the vents, a lot of air gets by even with the vents at 0%. Sealing off two of the slot vents from the outside with high-temp foil tape or simply stove cement and a cut up disposable foil pan will help, because if you don't, you'll have a hard time getting the temps below 240F, even with the lower vents at 0%. That's with blue-bag kingsford. If you burn lump or that KF comp ****, you'll be even hotter.
You can further drop the temp in the kettle converted to a smoker by choking the exhaust vent down, but that's considered a bad practice since it can create a smoky exhaust which will leave a sooty coating on the food.
A full fuel load of KF bluebag briquettes lit by minion method will run for about 6-8 hours. With firebricks, you can push them out a bit and sneak another hour or two worth of unlit briquettes in there when you are loading the fuel side. If you burn KF comp or lump a full load will run for about 2 hours less.
Here are some photos of the setup:
http://img53.imageshack.us/img53/6800/firebricks1.jpg
http://img39.imageshack.us/img39/6930/firebricks2.jpg
http://img33.imageshack.us/img33/8077/firebricks3.jpg
I used a 4-1/2" angle grinder with a masonary wheel to grind the oustide edges of the firebricks round to roughly match the curvature of the kettle.