Hi Peter. I've been using one for a while; in fact, I use it much more than my EQ6-R now. It was supposed to be my secondary mount, but that has changed. It's definitely the primary one.
The portability is one thing. That's fantastic. I no longer have to carry a heavy mount to whatever tripod I'm using. It also takes up much less room in the boot of my car! That alone is a winner with my family when we go camping, as "dad's stuff" no longer hogs every inch of available space.
I also had a night a while back where I didn't lose a single sub to trailing stars. I've never had that on the EQ6. People talk about random spikes in guiding in the PHD2 logs and that's true, but it has had zero effect on my images, and for the images of others.
I usually use 1 second exposures for PHD2 with everything else as default and it's working fine.
You also don't have to think about the "50-75%" rule (which is a rule no one seems to agree on) regarding OTA weight support. I run an imaging setup of about 17kg and I do use the counterweight extension. This isn't for balance, but for mechanical stability over the centre axis of the tripod. If you've got a solid tripod, I doubt you'd need the counterweight at all. When I get an adapter made for my EQ8 tripod, I'll test it out without a counterweight.
Any downsides? One or two, mostly software related that are and have been fixed. The first issue I had was in relation to the ASCOM driver, which wasn't performing a meridian flip at the right time (1 hour early!). I just happened to wake myself up one night camping and stopped the mount slamming into the tripod with mm to spare. With such a strong gear, your risk of damage with a strain wave mount is much greater than a conventional mount. ZWO have released an updated driver that apparently fixes this but I've yet to test it.
Also, I can't seem to manually update the GPS location all the time in the driver, if it doesn't pull it correctly from Windows. I use NINA to manually sync it instead. Apparently this is another thing that has been fixed but again, I've yet to test it.
I also haven't tried it with the ASI Air at all, though I should since that's why I bought it initially.
The carbon fibre tripod is nice, but do make sure you support it with some weight using the provided pouch. I have used my 17kg system on it and it was fine, but it does need some support.
I also have both extension systems and they are great for small scopes. If you have a longer setup, like my Esprit 120, then you'll want to wait for their new extension pier releasing soon.
I had Josh Bunn make me an adapter before I knew ZWO were releasing a new pier, but no regrets. Josh made me something that could probably support my house if needed
Polar aligning is simple too; the knobs to lock down your Alt-Az are solid, and the mount is easy to move. Do note if you plan to use it on an EQ6-R tripod that has an Azimuth bolt, you'll need to find a way to lock it down somehow to prevent rotation as that's what the Azimuth bolt does on the EQ6-R tripod. Josh Bunn's adapter includes locking holes for me.
Also, I did purchase an adapter for my Polemaster from Buckeyestargazer because using the TPAA in NINA is a bit frustrating. There's no way to natively mount the Polemaster to the mount, hence the adapter purchase. It now sits in the syta shoe on the side of the mount. Haven't tested it yet, as the adapter only arrived this week.
I haven't really used the hand controller so can't comment too much except to say it works.
All in all, I think it's an amazing mount. I don't balance in Dec or RA and that's awesome in itself. The OTA just gets chucked onto the top and away we go.
tl;dr if all strain wave mounts start off with this level of excellence and quality (didn't even mention the build quality, which is excellent!) then the majority of mounts into the future will be strain waves, IMO (except for observatory class mounts)
