Unless you're really know what you're doing, you do need a thermometer to get the best bbq results. You'll hear the real pros at barbecue saying they can tell the temp by the color of the smoke, or the smell of the smoker or even some more subtle cue. Until you get into it that deep, though, a bbq temp gauge is a must have.
You have a few options to choose from. Temp gauges can range from the 2 dollar home oven types to a mid-range $30 steel dial indicator or top-of-the-line digital remote units. In order to decide what's best for your own situation, here's some information on the various types:
Built in Thermometer - Dial indicator thermometers selling for around $30. Brinkmann makes one that fits a hole in the griller lid. They have a sensitivity of about 50 to 500F. The probe is inside the smoke chamber with the dial outside, therefore you don't need to open the grill to check the temperature (plus the gauge mechanism and face arn't exposed to the smoke). A few other instrument companies make dial indicator thermometers of the same type and you can get these on the Internet.
Digital Remote-Reading Thermometer - The Polder company has a good remote-reading thermometer for about $30. The Polder unit can be purchased at almost any barbecue supply store and over the Internet. These are electronic units that have a 4 or 5 inch probe you place in the smoker. A metal shielded wire goes from the probe to an LCD readout located outside the cooking chamber. You can put the probe anywhere in the smoker to get a quick, accurate temp reading.
Be sure and check your digital thermometer's calibration every so often. If you put it in boiling water it should read 212F or less if you are any altitude above sea level.