We use desktop VOIP phones, the mobile app, and the desktop app. They offer a "call park" feature that other system don't, allowing a parked call to be picked up from any other phone in the office, as opposed to other systems where you only put a call on hold and can then only pick up from the same line. The call quality is good, regardless of which access method used.
TERRIBLE support. Years ago, support was US-based, fast answer, easy to understand (native English), and always had the right answer. Reps used and knew the system. In the past years it has all shifted to Singapore where they are hard to understand, hardly understand your question, and don't seem to know the answer to anything without putting you on hold and going somewhere to look it up. Obviously a cheap off-shore call center.
I tried their support chat last time and it was worse - on hold forever and huge delay between replies while they obviously worked other chats.
The desktop app is pretty poor. They gave it a facelift, but its super slow to open and use - not sure what language they wrote it in. Half the time I use it to try to play back a voicemail, I have to hit the play button multiple times as it's unresponsive. The "new message" indicator never seems to clear.
I use their web interface to play back call recordings for training. Most of the time they won't play through their antiquated web interface, giving only a spinning "wait" cursor. Instead, I have to select a batch to download as a ZIP file and play them locally. Even when the web interface does work, there's no area for taking notes or anything to attach to the recording for future reference.
A colleague with a Mac can't even use the client anymore because it won't work on newer versions of macOS. Their support was unable to solve. He gave up.
I can no longer recommend 8x8.