I have a Keeley. Although I'm not currently using it in my current setup, the thing is amazing at what it does. It is one of the most transparent pedals I've ever played, when used sparingly, your tone will NOT be colored in any way.
In my opinion, it really excels in front of your amp on lower gain settings...it LOVES cleans. If you pair it up with something like an SD-1, you could have the right blend of overdrive and sustain for days.
With a Recto (I've got a Road King), you need to be sparing on the levels of the Keeley in the high gain regions as it can send the amp into a sonic frenzy. By keeping both the Level and Sustain closer to the 12:00 regions, the Keeley works very nicely. I preferred using it with the Raw mode only because by nature that mode is less compressed to start with. During a solo, I'd kick in the Keeley on that mode and it worked VERY nicely. On the Vintage or Modern modes, keep the level as close to 12:00 as possible and the Sustain below 3:00. If the Sustain control goes above that setting, you could have a hard time controlling feedback with any kind of volume driving your amp.
In the right app, the Keeley is great...check one out on the clean setting of a Mesa and you'll really dig that snappy, country chicken' pickin' type tone. Put it in front of higher gain and you can get some sick sustain once you dial it in right. It won't be harsh at all...as with anything, you get what you pay for.
Erik