I've got an MS-3 and have found that it's useful in a lot of situations, but does leave a bit to be desired.
- The overdrives are surprisingly good, but they aren't tweakable on the fly, which for me is super important in live situations. I find that tough rooms have a big effect on distortion sounds so I generally keep my OD and fuzz in the loops when I need them. I have had good success with recording those effects though.
- The compressors are fairly nice but I can't bring myself to ditch my Seymour Duncan Studio Bass. It lives on my board in front of the MS-3.
- Pitch effects: the bass octave is decent but it's very clearly digital. I like my analog octave pedal in the loops, personally. The pitch shifter is decent in Mono, and I've used it on several occasions to emulate half step or full step down tuning for one song in a set. My favorite pitch effect is actually the Overtone, which does a cool organ thing when combined with some drive.
- Modulations: I like the choruses, phasers, and flangers with a clean blend, though the phaser isn't quite as deep and rich as my Phase 95. The vibrato is really good, I like that one a lot.
- Reverbs and delays: these are doubtless the best part of the MS-3, effects wise. They're clear and pretty easy to use.
My biggest complaint with the MS-3 is the lack of a universal clean blend or parallel processing: if I could run dry to one output and wet to another I'd be thrilled. I'd be even more over the moon if I could assign some effects to one channel and other effects to the other. But alas, it's not quite there, and the one time I managed to reach Boss customer service to ask if they planned on updating the software at any point, I got a surprisingly curt message that amounted to "spend more money or piss off," so I'm not holding my breath.
The MS-3 is a good value for money and a great way to tighten up your board if you want to spend some time with it, but it ultimately falls into an uncomfortable middle ground for me. It's not quickly tweakable and simple enough to be efficient for most of my gigs, so for those I use a small board with 4-5 pedals including a tuner. But it also isn't fully featured enough to fully satisfy my needs as the "brain" of a larger board. It's close enough that I'm holding onto it for the time being, but there are days when I just want to sell it.