I have two of the Zoom MS100 BT units. If you have an Apple iOS device you can add a lot of FX which are omitted from the MS50, and incidentally the 100 has stereo input should you need it.
The Zoom spacial, modulation and compression effects are excellent. The enhancer is worthwhile and the noise gate options are not bad. The amp sims and distortions are very good although prone to a slight high whine around 5khz when gain is high which on occasion bugs me. The parametric and graphic EQs don't get my mojo working, I get on much better with dedicated pedals which seem to have more control and more gain. Indeed, I don't know if it's a hardware or software issue but not every effect offers all the sheer gain I would prefer.
If set to a single effect, these Zoom pedals work pretty much like any other stomp, and the foitpedal toggles On/Bypass. Alternatively you can chain up to six effects and toggle between sets of effects (though the single switch means you can only cycle through in one direction). For instance I have an overdrive chained with an enhancer and noise gate. I have other two identical effect chains except that each has still hotter overdrive. Thus, hitting the footswitch is like cycling through three amp channels. On occasion I add in EQ (such as to add a mid-boost to the high gain 'guitar solo' chain) or a compressor (to make solos 'sing' more through sustain). My other Zoom 100 is in the effect loop where it provides reverb and then adds in Univibe or chorus if required: I put a bit of noise gating in there too. It's a unique benefit of the Zoom pedals that they can run a chain of effects, and I use the facility more than I expected once I found the benefit of tweaking the "main" effect with additional EQ, compression, gating etc.
The Zoom has tap tempo but I never use it as you have to hold down the switch a while to trigger it and 1. that's a bother 2. opting for tap tempo disables triggering the tuner with the footswitch. Thus, if tap tempo is a big deal for you, the Line 6 M5 may be worth considering given that it appears to have a second dedicated footswitch. The Line 6 also can take an expression pedal, albeit one of Line 6's non-standard ones. The Zooms have no expression pedal socket. I have read about some reliability issues with the M5, though that may have been early production. I'd not buy one used!
EDIT: LOL, I've just noticed this thread is a year old: I saw January and though it was new!
EDIT 2: the iStomp is the most basic by far and useless without an iOS device, but they are now stupid cheap (check somewhere with fresh stock: literally about 1/3 of the launch price) and all virtual pedals bar one are now free. I'd still not buy one over the competitors, but they appear to have a lot of gain on tap, and I'm half tempted to add one to my Zooms at the crazy low price on the assumption I'll find some good pedal to set it to permanently. Heck, though, then I reflect that I could double my outlay and get an M5 to play with...