I recently bought a 707 and there's a lot it does well.
The stock waves and sounds are very well done, though browsing them is a drag - the patches can be browsed by type (bass, lead), but if you want to find a particular waveform, good luck,
The pads are a nice 'halfway house' style suited to drums and instruments, closer to Maschine or Launchpad than MPC, and the sliders and knobs work well. Some of the shortcuts are a little vexing because the position of the Shift button means you need to use both hands - I prefer the double-tap setup of my MPC One here.
In terms of inputs and outputs, it's very flexible - with the current OS you can use it as a four-track mixer / live sampler / looper / FX box, with four tracks of internal sounds (and of course you can sequence the devices that are hooked up). Or you can have a hardware send and return path alongside the two inputs. It beats the One here for sure, though the One has the upper hand with USB devices and MIDI routing.
Sound design options are very broad with a strong selection of generally very good effects, though it can't compete with the depth of the One here - certainly no per-pad FX, and just one FX slot per program. But what's there sounds good, and it's easy to set up nice dedicated controls for the master FX. An advantage here over the MPC is that you get a solid core setup for quick results (but nothing you couldn't fix with an MPC template).
The synth engine is powerful and deep, and they've done a good job of making it work on the small screen with limited controls. It's not as immediate as the MPC's plugins, with their dedicated interfaces, but once you know the layout it's probably a little faster using the 707 - the UI is consistent and logical, and although some menu-diving is unavoidable if you want to get your hands dirty, it doesn't feel like a chore. The VA quality feels about the same, and the rompler synth features are more effective than you might think if you haven't used a groovebox since the 90s...
The clip arrangement style is easy enough for anyone with Ableton experience, and works well for a small screen / device. Giving you two mute pads per track - synced and instant - is a nice touch. The Scatter effects, which haven't usually appealed to me, are interesting here because you can customise the effects on all 16 pads, which means you can craft some quite subtle and useful effects - or you can sequence any of the individual parameters on a 16-step pattern, which you can modify in scale or speed.
There are some annoyances. My biggest bugbear is that the sample and hold LFO appears to just be a static sample slowed down, and if you sync it to a keypress it starts from the beginning every time, so you get a predictable pattern. So you can't, for example, use a slow synced S&H wave to alter the filter on every note. I'm at a loss as to why I didn't see this mentioned in any of the videos and reviews I checked out. And the other big one is that their fairly new clip chain feature is of no real use, because changing a clip cuts off any audio tails from the clip before - again, I haven't seen this mentioned anywhere, but I have had it confirmed by other users.
I'm sure that'll be fixed, and I'll submit a request to do something about the sample and hold. But otherwise, I think this is a good device, particularly as a kind of 'Roland Museum'. I don't think it's a One / Live killer - there's a different workflow and the sampling features are limited and a little clunky in comparison (and sample time / audio tracks is a big positive for the MPC) - but it could be bad news for a few of my other samplers. It'd be interesting if Roland developed some of the features into a more focused sampling device, like an MV update - then I think there'd be some serious competition.
I will also say that although I was skeptical of the MPC touchscreen approach, I have found myself jabbing the 707 screen a few times...