Over the past 10 years or so, I've probably spent 15-20 days on the Deschutes, but most of it has been from mid-August through early October.
So, I don't know much about the fishing in May other than if you're out there late in the month, you might get into the salmonflies on the lower river (below Pelton Dam). This is the big water section that Tom advised you to get a boat for, and I think he's probably right for the most partbecause I don't know the average differences in cfs volume down here between say, May and September other than to assume the water is lower in September.. I know I waded a lot of the section between Mecca Flats and Trout Creek (the upper portion of the lower river, if you will) and was ok so long as I chose my spots. I had real good fishing throwing elk hairs and other high floating flies up against the bank. Even if you don't run into the salmon flies (and you may be blessed not to because they draw the biggest crowds of the season, the place is a caddis factory. I've never been there that some sort of caddis or another wasn't thick in the air. These fish see a lot of flies, but aren't really that selective, IMO. An assortment of deer or elk hair caddis in 12-18 with brown, tan, olive and cream bodies (peacock too) will take a lot of fish.
If I were you, depending on where you are basing out of, I'd take a look at the upper river from Bend down for about 20 miles. Whether this is worthwhile will depend upon the irrigation drawdown schedule and I don't know what it is like in May, In the Fall, it makes for very pleasant fishing. Anyhow, this section has limited access, but there is good fishing (for browns and redsides) where you can get on it, which is at several state and county parks from Bend down a ways, about to a place called Tetherow Crossing.
Here is an excellent book that covers the entire river, section by section. It's very worthwhile to have if you're going to be there long enough to invest 17-30 bucks in the book. It also covers virtually every other fishery in the area and some of them are pretty formidable.
http://www.amazon.com/Fishing-Central-Oregon-Beyond-5th/dp/1882084136/ref=sr_1_1?ie=UTF8&s=books&qid=1264464592&sr=1-1
Finally, if you're looking for a small stream experience at some point in your trip and don't mind catching little fish, look at Tumalo Creek near Bend. And if the Deschutes is carrying too much water or the withdrawals have it messed up, give some thought to skipping over to the Crooked River in the high desert about 30 miles east of Redmond. It's an excellent redsides and whitefish river about a quarter of the size of the Big Deschutes and at one time it was carrying 8,000 redsides a mile. I think it's less now, but there are still a mess of fish there and they really aren't hard to catch.
Good Luck..
