Pretty much any clean boost will do, I personally use a Seymour Duncan Pickup Booster because I picked it up for cheap, 50 bucks. I'd love to have one of those tubescreamers and get it modded, but man its just not worth the extra hundreds of dollars to me. The SD booster is just a clean boost but its a good one, and it doesn't suck the tone.
The way its been explained to me is, the pedal is used to hit the preamp harder at the front, which is different than just turning up the gain (the gain knob sends the signal to the power amp), and produces a different type of tone. I guess its like using higher output pickups, but you choose when to boost the power and how much to boost it by.
I'm also glad I opted to get the 2x12 3/4 back cab instead of the Recto or Roadster cab... it sounds much more open and I don't find myself needing the massive low end punch that the Recto cabs provide.
Go Roadsters! Its funny, because it seems like pretty much all of my tonal fixes come from a combination of: turning the treble down, turning the gain down, and turning the master up. The treble and gain knobs really are the keys to tone tweaking, especially the treble. You HAVE to move that treble knob around, because it adjusts the balance between treble and mids (bass I don't care so much about, just adjust it to taste), and the treble/mid balance is really what sets tones apart, in my opinion. And the treble knob is SO sensitive, you really have to keep moving it in teeny increments to hear what its doing. I used to try and adjust my EQ by moving only the mid and gain knobs, or by moving the various knobs a lot, but now I know you have to move the knobs bit by bit because there are so many tones in there.
I also use my output control to set my volume so that I can crank my masters to 12-1 ish without blowing my band away. It really helps me get the nice saturation I like, though my cleans suffer a bit. Recently I've started setting my Solo control a little louder than my output, so now I just use that to boost my clean's volume a little when I need it.
It also seems like I prefer lower gain settings than a lot of people on here. On my Modern channel 4 my gain is around 10 oclock and my master at 12:30. But I get my extra gain from my pedal, which I guess is why if you don't use one you would need to turn the gain up. It just sounds so much fizzier to me.
Comment away, oh lovers of Roadster!