I did, once, for a year or so. It was around 2004, I think, and what caused me to transition was a slightly too big number of bugs in FreeBSD at the time. And not too good support for USB and sound, IIRC.
OpenBSD was stable like mountain, rock solid, ran xfce (my second favorite WM) without any problems and was even better documented and even tidier in it's design than FreeBSD. It supported all of my hardware at the time, the sound card was enabled by default and the ports were in much better shape than in FBSD - they actually compiled, every single one that I tried, while FBSD had quite a few broken ones. Overall, the experience was very good and I migrated back to FreeBSD much later than I was able to (ie. when all the issues were fixed) just because of this.
So no, I would say that this entire thread is missing the point - OpenBSD has a potential to be a good desktop OS, at least at one point in time it actually was a good desktop OS and it could benefit from being one once again (if it somehow ended not being one now, I don't know about present).
I hate Linux with it's numerous distributions with passion - any BSD is just so much more elegant, consistent, intuitive (probably I just got used to them) and clean - and I won't use Linux anymore (I did, for a few years back when Y2K bug was popular). For people like me FreeBSD is an obvious solution, but if it irritates me again I'll look in OpenBSD direction. I'd be very happy if it was ready to accept me as a desktop user then.
Oddly enough, I switched from NetBSD as when its iMac support for X was broken for several months, and I haven't switched back. (For the uninitiated, it's NetBSD that is supposed to be the one that runs on anything.)
I really like having it around to do things like experiment with pf, and test C development in a non-Linux environment. Especially for C for one who is used to Linux, it's a bit of fresh air on OpenBSD.
I also have a few VPSs around, including one with FreeBSD. I wish more of them offered the BSDs.
I have never, once, seen OpenBSD run as a Desktop OS. This despite working at several companies where OpenBSD was actually core to our entire infrastructure.
I'm not saying that nobody does (I expect the OpenBSD developers probably do), but I am strongly suggesting that OpenBSD needs no desktop presence whatsoever to be extraordinarily successful. I've never installed it on the desktop. I've never seen anybody install it on the desktop. Not once in 15 years. There is no rationale reason to run OpenBSD as a desktop, and I'll go further and say I will likely never see X-Windows run on OpenBSD.
And, as for a niche - I consider OpenBSD to be the pre-eminent platform for Firewalling, IPsec, IPv6. Their continual focus on consistency makes the system a joy to use - all of the lessons I learned in 2001/2002 are almost 100% (hell, they might be 100%) applicable to managing an OpenBSD system today.
They may be a niche but it's a pretty darn big one.
With regards to your argument regarding "upstream" support - most of the upstream support required is around elements that are not associated with the desktop, and, in many areas, openBSD has just decided to rewrite the upstream contribuions in their own "OpenBSDsh" way anyways.
I'm confident they will do fine even if upstream support for desktop, and associated applications, no longer considers BSD as a first-rate platform.