They were cool.  However...

I returned it.   Sticking with my trusty GT-10 instead.

Here's an excerpt of my review from the Boss forum:

In the beginning, it seemed to me that there was a significant update to the COSM amp models in the 100. I'm not so sure about that anymore. There might be some new life in those "Advanced" amp models in the 100...harder to tell than I originally thought.

All of this doubt started creeping in when I copied my patch settings over from the 10 to the 100. Those patches for my rock band are using the OD models to go into the front of a clean amp. Using the same settings on the 100 as I did on the 10, the sound was 100% identical. I A/B'd back and forth at home before rehearsal and also didn't hear any difference at rehearsal with the 100.

This lead me to go farther and start investigating amp models. Those not designated as "Advanced" have definitely NOT changed from the GT-10. Even the "new" "Natural Clean" amp sounds identical to the "Boss Clean" to me, although there might be some slight differences for higher values of the gain knob. I'm going to go through the rest of the advanced amps and try and match them with existing ones on the 10 because I have this sneaky suspicion some of them are just renamed in the 100. The OD models, despite some new looking names here and there, are also 100% unchanged from the GT-10.

Sounds aside, I noticed some minor annoyances from a performing perspective with the 100.

LEDs and Effect Indicators. The LEDs on the switches in the 100 are about 1/3 the brightness of those in the 10. I know a few people complained about the ones in the 10 being bright but I actually liked them that way. On a dark stage with the 100, I had a pretty hard time seeing the pedals and their indicators (more on that in the next section). In bright stage lighting or sunlight, the pedal LEDs were almost impossible to see. As far as effect indicators go, there are no LEDs for these on the 100. You can switch your screens to a view that shows effect status, but it's not as visually eye-catching as the bright LED indicators on the 10.

Unit Color. Sounds like a silly thing to complain about, right? Picture this: Black MFX pedal with black switches sitting on a darkened stage with dim LEDs. Extremely hard to see the pedals. You can turn on the pedal indicators to flash in the Play Option menu. This helps a bit, but it's still not enough. Some fluorescent gaffer tape might help on each pedal, although that still wouldn't provide the visual contrast that a metallic silver pedal with contrasting black switches (and bright LEDs) has.

Screen(s). Okay, these aren't too bad. The new color scheme has sufficient contrast and it readable to me in almost all lighting. I can't really complain here. I do prefer the bright blue of the 10, though. Maybe my eyesight is failing with age. And it is kind of cool to have one screen dedicated to the effects chain while another shows parameters. The editing is really simple on this unit compared to the 10. Still, I kinda miss direct access to each effect via a button. I know this ties the hardware to the software and doesn't allow for changes, but honestly, have you known Boss to change anything once released?

Manual Mode. The thing I like most is that manual mode assignments are saved per patch. That's very cool. What isn't cool is that your Accel/CTL pedal assigns are not available in manual mode. In almost all of my patches, that pedal is assigned the OD/Solo function and additional assigns to turn on EQ, switch delays, and maybe even comp. It's my lead boost / solo settings in a single stomp and I like it that way. To get around this, I had to put an outboard FS5U and assign these functions to it. Problem solved, right? Not really. Now I can't use my external expression pedal for volume. And I want to do this because I have my wah set to auto-engage on every patch and that means I can't use it as a volume. To add insult to injury, the way I've necessarily laid out the FS5U out in my case with respect to everything else makes me either switch feet to engage the solo or do an awkward balanced reach with my right foot. Makes it hard to get at the wah pedal quickly too. In the 10, I'd have the CTL2 as my solo switch, right next to the wah and easily accessible to my right foot. Also in the 10, the two onboard CTL pedals retain their function in manual mode. Granted, I don't have the flexibility of saving manual mode configuration per patch, but I've also found I don't use the unit that way.

Phrase Loop Switch. The good news: You can disable it from turning on the looper and use it for assigns. Bad news: 1. You can't assign it a dedicated function like you can the Accel/CTL switch. 2. The LED doesn't light when an assign is engaged. Might that be a bug? Maybe. But if it was, it wasn't addressed in the firmware update.

Bank Switches. They're on the left now. I like switching things with my right foot. That means that same awkward balanced reach across the board to bank up, down, or engage manual mode, or doing a two-step jig that makes me look like a guy from Riverdance. Disengaging manual mode by pressing the two of them also brings in the possibility of missing the synchronized press and turning on or off an effect you don't want turned on or off. Of course, the manual mode problems could be fixed by assigning manual mode to the phrase loop switch in both regular and manual mode.

Spillover. Initial response seem to indicate that this feature is gone now. If so, major annoyance for me. I use that feature a lot and it sounds really unprofessional to me to have a prominent delay sound to just abruptly cut off when I change patches. I can't believe they dropped this.

Bottom line, while the unit does sound good, I don't think there's much in the way I use the GT that has changed for the better (for me) in the 100. I'm considering returning it and continuing with the GT-10.