As with any other rating, you can always give it a 0 or a 10. I'll explain myself. You really need to know what you are getting into when you buy this gear. As of today, this is unit has no competitors out there, being a hybrid of a pedal board and a multi-FX processor. The pedal board is easy to use (with few knobs like on a classic FX pedal) and it has an auto-save function (it seems the unit retains a snapshot of the pots position). The multi-FX processor can save several presets of the same pedal, it allows you to freely place the effects of the loop according to the "scenes", it offers the possibility to have 12 pedal boards with 12 presets each, it allows you to use one or more expression pedals to control any parameter, and you don't have to worry about wiring problems. Don't be misled by the ease of use because the possibilities are huge. However I do miss a bypass button because it forces you to create an empty scene for that purpose.
It is very easy to use, there's nothing special to say in this regard.
It must be clearly stated that this is a digital unit. From my point of view, the comparison to vintage effect pedals is inappropriate, even though Line 6 based its marketing on this assumption. Line 6 engineers replicated the operating modes of the pedals rather than the sounds themselves. The sound is pretty similar, some emulations are even very close (judge for yourself in this video of the DM4, which is a section of the M13) http://fr.youtube.com/watch?v=eCi48KHyiek.
If you are looking for a vintage sound, go buy exorbitantly priced, super rare, and less and less reliable one-button effect pedals. Otherwise, if you want to move along with your time instead of thinking we are still living in the 70's, you'll have fun with this one. What you get is different TYPES of effects, old and new, without being an exact copy of Hendrix and others. So your sound will be... new (oh my god, that's awful...!) All in all, I think Line 6 does much better on digital than lots of other brands. It sounds less like worthless pop music for supermarkets. Line 6 effects are trendier than those of TC electronics or Lexicon for example.
I've been using it for 2 weeks now and I didn't really need any time to get used to it, it's really easy going.
It's an hybrid between a pedal board and a multi-FX made for people somewhere between typical pedal board users and rack-processor users, who might be bored, but are not quite ready to make the leap from one extreme to the other. Take the time to read about it before you buy it and you won't be disappointed. If you are, then... you don't know how to read. :D