Just did this trip last weekend, so I can now answer my own question - Yes! the 15 miles on Poison Springs road and the 12020 mining road would likely be 3x faster than in my full-size 4WD truck, but it would be harder to haul gear. A side-by-side ATV might be best. Here's a few of my observations:
1. Happy Canyon is well worth the effort to get there, but it takes quite an effort to do it in a day. We left Hanksville at 6am and returned by 6pm. We were at the mouth of Happy Canyon at 11:45.
2. The road has been cleared of most boulders; there was only one I got out to move, and one more that was too heavy to move, so I drove over it. I made it all the way to the boulder slide with my full-size, stock-height 4WD pickup, but it would have been better to stop a little short at the mesa camp site - that last quarter mile was a bit hair-raising!
3. Mountain bikes are useful to save time getting from the boulder slide to the downhike to Dirty Devil, but its just as much work as hiking. We ditched the bikes about 2/3 of the way there - just too many land-slides and wash-outs to carry the bikes over. So if you have an expensive, light-weight, full suspension bike and are good at technical riding, give it a try. Otherwise, leave the bike home.
Attachment 88544
4. There is a wicked, ice-covered pond across the Poison Springs road at the 8.3 mile point. After driving through many icy puddles up to that point, this looks like just another - but it will swallow your vehicle and hold it captive! We took three vehicles: my 4WD Duramax pickup with oversize tires, a totally stock 4-door Tahoe, and a RAV4 suv - we all got stuck in the pond, up to the axles! Good thing I took a tow strap! I pulled out the RAV4, the Tahoe pulled me out, and the Tahoe manged to somehow get himself out. Its covered with ice about 3.5 inches thick - you ride on it until you break through, then you are stuck real good.
5. Best place to wade the Dirty Devil is just downstream from the mouth of Happy, where the water is more swift and shallow. In places that the water moves slow, its deeper and has quick sand. Good to have treking poles for stability and to probe ahead for hidden holes or quick sand. Water is color of chocolate milk, you can't see through it.
6. Its worth it to hike to the top of the canyon - some nice up-climbs to practice your stemming there.