The Sandy is a river that I have fished this river only twice and I found it to be pretty tough to discover via the road program. Not that there isn't great water to fish that is accessible by road, It is that the dense forests and the question of private vs public property makes knows where to go and finding a way there tough. So over all, I think the best way to discover the Sandy is with a guide. With a guide you can float the Sandy which I think would be the best way to see it.
Two good friends of mine , Tony and Rusty, floated the Sandy with Tom Larimer out of Hood River, OR and they loved it and learned lots. They were new with two handed rods and got lots of casting instruction from Tom, who really is an expert, and they were able to get this education while fishing great runs and searching form big fish.
I have not floated the Sandy, I have gone with the road program. I have found some great runs, been told of a few others and the waters of Oxbow park are pretty obvious. I have not caught a steelhead on the Sandy, I have hooked some, I have had bone jarring grabs, but no fish.
I know that the Sandy has a summer fishery, for both salmon and steelhead, but I have not fished it.
The times that I have fished the Sandy, I have primarily fished a 13'3" 7wt Burkheimer (which is my favorite rod for everything but small fish) with a 540grn compact skagit, but I think that alot of the runs on the Sandy would fish a longer head 55-68' well, like a Delta or a PowerSpey, especially on the runs around Oxbow State Park.
There is a really major spey clave held at Oxbow Park on the Sandy river each spring, because of this the Sandy is really well known among spey guys. The Flyshop in Welches is a good place to get some info and shit, they also have guides, but I know Tom Larimer and he is who you should get if you get a guide.
When I have been on the Sandy, Oxbow park has been my point of focus. This is mostly be cause there are some pretty obvious runs in the park and there is good camping as well.
As with any time that you are fishing for Spring/Winter Steelhead be ready for swiftly changing weather. Rain jackets are a must, cleats are a plus. Another key piece of gear, a headlamp it never hurts to be the first through the run.