xscottx9's quick and dirty reviews are also fairly accurate IMO, with maybe one alteration. The OCD overdrive, while nice enough on a clean channel, is after all an overdrive, and sounds much better in front of an amp's dirty channel. To add even a bit more clarity, I will mention that IME, it depends quite a bit on what kind of amp you are putting it in front of. That's true with any overdrive.
For instance, the OCD, on pretty much everybody's short list as one of the best overdrives ever made by anybody, is very Marshall-y sounding and provides a somewhat full-range tone. Playing it in front of a Marshall or a Stilletto will do different things to those amps' sound than playing it in front of a Recto. An OCD in front of a Recto tames and tightens the muddy bass, puts back the sweet mids, and rounds off the harsher, fizzier highs of the Recto. IOW, it sounds fantastic....better than either the OCD or the Recto sound alone. In front of a JCM-800 or a Stilletto Ace combo, it will give more girth to the bass frequencies and fatten up the mids and highs, compensating for the sometimes thinner sound those EL-34-based amps provide naturally. The same could be said for other full-range boutique overdrives like Fulltone's Fulldrive 2 or GT-500, or the Barber Direct Drive, or the Xotic BB Preamp.
Taking a look at an overdrive like the Tube Screamer or its variants (808, TS-9, Maxon, SD-1, Zakk Wylde OD) that cut more bass and tend to emphasize the upper mids, these overdrives tend to sound better in front of an amp like the Recto, which naturally tends to have a little mid-scoop and a little mud in the bass. They can, of course (and were for years) be used in front of Marshalls or Marshall-sounding amps like the Stilletto, but for today's guitar sounds, there may be too little bass and/or the upper mids can get out of hand.
Now, the modified versions of some of those Tube Screamer-based overdrives tend to give a more full-range response than the stock pedals. They can still produce the sounds of the unmodified pedal, but they can also provide a surprising amount of bass, and can be adjusted to leave out the mid-range bump they usually provide. As such, they can be used quite well with either a Recto or a Marshall-sounding amp.
Clean boosts are a different animal. They give you essentially more of the same sound you're already getting. In front of a Stilletto Ace combo, your mids will fatten up, but you still may not be getting all the low-bass you're wanting, and you may need to dial back on the treble to avoid that amp sounding strident. On a Recto, a clean boost won't give you the mids you need for soloing, and it will only make a muddy bass sound get muddier. Fizzy highs will get even fizzier. The point is, with a clean boost you need to make your tone sound as good as possible to start with, then the clean boost will just give you more of that good tone.
stompboxfreak72 brought up a good point about the Digitech Bad Monkey being a good pedal for the money and often overlooked. Unmodified, it's a more versatile pedal than the unmodified SD-1 since it gives you more control over the bass response especially. However, where that particular pedal is an especially great value is if you modify it a bit to give even better bass response, wider dynamic range on the gain control, and more singing mids. With just a few very cheap DIY mods, that pedal can get you some tones rivalling boutique pedals that cost $100 more.
