Boss says, the OD-3 had a "bigger, smoother tone than any Overdrive pedal ever had". I say: It's true!
The tone is very warm and fat, much smoother than for example the Boss BluesDriver. The drive range starts with a nearly clean, slightly fattened tone and ends with a solid rock sound. The tone poti allows to adjust the highs and the "punch" of the Overdrive. It's a good way to control the basic sound of the drive, the overall control still can be done with the amp.
The OD-3 fits perfectly in any amp character: It supports its tone and doesn't change it too much. It always adds smoothness and bass to it, but if you have a Marshall, it will still sound like Marshall. The best thing is the OD-3's dynamic: You can turn the drive poti completely up, but you'll still be able to control the drive from clean to completely overdriven just by using your guitar's volume poti.
By turning down the drive knob and cranking up the volume you can use the OD-3 as booster and sustainer too add a creamy component to your amps overdrive. I use it with volume at 10, drive at 2.5 and tone at 4: With a clean amp sound I get a nice blues tone, with my amp distorted the Boss is a booster for my solo sound.
Quality and handling is classic Boss-like. Perfectly crafted and a real work horse on stage: You could drop this thing down from the 10th floor and it would survive :-)