Critter,
I'm not familiar with the last two that you already have. The Keeley SD-1 5-Star mod is one of my favorite pedals. To me, it's about the best you'll ever get a mass produced overdrive to sound. I own all three of the pedals you're debating between.
The OCD is almost like having a Marshall Plexi on steroids with extra gain and bass, all in a box. There's a good reason why it's on most people's short list of best overdrives. The harmonic overtones it generates are truly spectacular. Imagine being able to play leads through a Recto, only getting lots of sustain and sweet singing feedback that you don't get with the amp alone.
The Fulldrive 2 is also a very good one, but IMO its less versatile due to its darker tonal coloring. When paired with a Recto, the darkness of the pedal will have to be tamed or it will combine with the naturally woofy bass tone of the amp and bathe your guitar in mud. However, that very same darkness can serve to tame the glassy highs that Rectos produce on Channel 3 Modern and give your leads not only ample sustain, but a much more listenable warmth in the high notes than the amp itself can give. Where the Fulldrive 2 is almost miraculous, though, is when it is played by itself through a good tube amp's clean channel. It will give you a warm, full-bodied blues/jazz tone that you almost won't believe. The pedal by itself sounds like a better light-to-medium-gain tube amp than many tube amps do....it's that good at that particular type of sound.
The GT-500 is a pioneering pedal in two respects. It's the first pedal to use a wah pedal's midrange circuitry to provide the widest range of midrange boost/cut control of any production overdrive ever made. It's also the first to give you the option of choosing not only independent on/off for each effect, but also the ability to reverse the sequence in which they affect the signal...IOW, which one comes first. To be candid, I have found this feature to be completely useless and gimmicky. Overdrives always sound better before distortion, regardless of whether that distortion is generated by a pedal or an overdriven tube amp.
The overdrive in the GT-500 is very good. It's closer tonally to the Fulldrive 2 than to the OCD, but it does share characteristics of both. It offers treble and bass tone controls, which is a very nice and useable feature. With the gain way down, it offers outstanding amounts of very clean boost. With the tone controls, you can further shape the tone that's going to overdrive your tube amp, and this makes the overdrive side versatile enough to use with a Marshall Plexi, a JCM-800, a Mesa Stilletto, or a Recto with equally good results.
The distortion side I have found wanting, frankly. The basic tone is surprisingly nondescript and sounds very much like a generic distortion pedal. I have several other analog distortion pedals, all of which were much less expensive, that I would choose over this one any day. The midrange control's huge dynamic range is interesting, but not nearly as useful as it could be if Mike had allowed you to choose which midrange frequency you want to adjust. Instead of so much choice of cut/boost, I'd rather Mike had given you about a 12 dB cut/boost, but allowed you to dial in which frequency. The one provided in the pedal isn't really ideal either for nailing the mid-scoop sound nor for boosting the sweet singing overtones into your leads.
Overall, the GT-500 is a very well-made unit with great customizable overdrive tone, but a distortion side that is, again, a bit gimmicky and also a bit sterile. As much as I love the overdrive side, and as much as I want to really like this pedal, it's not one I would buy again if it were lost. Unless you're really wanting a distortion pedal also, and not just an overdrive, I would not recommend this one to you.
Of the three you mentioned as options, I'd say the OCD edges out the others. It, too, will have to be tweaked to your taste, since it also offers a truly generous amount of bass response for a pedal, and that could be a problem if you use lots of bass on your amp already. However, with the tone control set higher and the mid switch set to "HP", it will clean up just fine with a Recto. The Fulldrive 2 is a bit trickier to dial in when using it with a Recto with its bass set high. However, it can be done, and it would seem a shame to miss out on those really beautiful warm/clean sounds that it can generate. Ultimately, I copped out and bought all three. :mrgreen:
