I have irrational bias against Boss guitar gear - just thought I'd say that up front.
This is a fantastic bit of kit if you're the sort who isn't frightened by compact gear; there really is just so much thats useful to a guitar player in the BR80. I had the BR600 previously but hated it; I thought COSM sounded crap and the interface was horrible - way too much messing around with menus and sub menus on a tiny display. This thing is the total opposite; it's very, very simple and intuitive to use and the DSP chip has been updated which has made a marked improvement to the quality of the patches, of which there are tons and all of which can be edited.
It's a bit bigger and thicker than a typical smart phone - you'd need a fairly big pocket to carry it around it but it's still very portable.
For practice, there's 'eBand' mode. The unit is supplied with a ton of loops in loads of different styles but you can also upload your own audio files, backing tracks, and with with the Centre Cancel function, you can upload original tracks and minimise the volumn of the solos, change the tempo and key. It even suggests a patch to go with the type of loop you have selected (which can be overridden with your own choice) You can also loop AB points. The tracks can be organised by artist, album etc. as well as a '30 Favourites' list which you can change as you want. Loops, backing tracks, etc. are imported/exported via a little program supplied with unit. Performances can be recorded and ported over into the MTR mode so that you use all the MTR tools to mess with your recording. You can also do this with the Live mode.
In Multitrack mode, you have an 8 channel recorder with 8 virtual tracks per channel with all the standard stuff like a drum machine, programmable to a decent extent, Low/Mid/High EQ, separate reverb for each channel, panning, Bass and Vocal effects, pitchshifting if you don't have a Bass guitar, mastering tools, etc. with graphic 'faders' and 'pots'. To be honest, I've only messed aroung with the MTR mode so far but it's clear that it's very easy to use and is something that I definitely plan to get around to using.
Finally, theres the Live field recorder mode which works via the onboard stereo mics. I use this mode for recording and listening back to band practice, recording lessons, etc. The mic sensitivity can be adjusted, as well as the inport and output levels and the mics themselves do a good job.
A unexpected bonus is that it can also be used as a USB interface. I tried it with my HP Windows 7 laptop and Reaper and it worked, glitch free. Getting my UA25EX to work with my laptop was a total nightmare but the BR80 worked like a charm and without any issues at all, after installing the driver.
No bit of kit is perfect so here are the downsides that I've found. In Live mode, you can only record with the onboard mics - the guitar input is disabled, the oboard effects can't applied to the 'Line In' signal and that the time signatures is limited to 4/4. None of these issues, though, even come close to being deal-breakers for me, and they could end up being sorted out in a future firmware upgrade. I know that various forumites have been pushing these points to Roland so they may indeed get sorted out.
The last downside is the price of the BR80's optional silicon case and soft neoprene bag. Seriously Roland, 25 sheets?! You're having a laugh! I bought a external hard disk case to protect mine when I chuck it in my guitar case, gig bag or whatever and it does a much better job of protecting it from scratches, impacts, etc.
All in all, a brilliantly useful bit of kit that's genuinely easy to use, bunged full of really useful tools and sounds great.
