I have a 20" BC wheel and a 24" ultimate wheel. I haven’t spent much time with my BC wheel but can ride it down an incline at a pretty slow pace for 100-200 feet. It’s fun but not great transportation. You really want to ride it downhill which means a lot of walking back uphill. Nicolas Potier’s is my favorite BC wheel video and shows what’s possible at the higher end of skill and experience:
For me the ultimate wheel was one of the easiest things I’ve learned but it’s physically more arduous. It’s a great workout! There are some awesome videos of George and Kris Peck riding ultimate wheels in ridiculous conditions. In fact they are more correctly wheelsets which is more difficult to ride than a “regular” ultimate which has the pedals closer together. While basic riding is easy to learn there are incredibly cool tricks you can do if you have better coordination than me. Some of my favorite ultimate wheel videos are from Anthony Soumiatin and he has a great ultimate wheel playlist:
The first time I took my ultimate wheel offroad I popped a tire so have been reticent to take it back on the trail. It probably would be great training for mountain unicycling (and riding in general). It’s not bad transportation since it’s even more portable than a unicycle, but it’s much more tiring. I’ve taken mine on a pub crawl. $250 seems a bit steep for a BC wheel.