I fish it every year, gives me a place to Spey cast in the northeast. The brood stock fish are aggressive but in spring the water levels are too high for good shore fishing. This year may be different.
There are three areas, two old and one new. Sewall's Falls area is the most commonly fished. Then you have the Franklin stretch. Third is a new area above Franklin, just opened.
You should contact John Greenwood at the Concord F&W headquarters and attend the open house they hold each year. I'll be up there, possibly with a few other Forum members.
Fly patterns seem to morph by region even for the same fish species. This is mostly due to the fishermen, not the fish. The local favorites are very bright marabous and bunnies like winter steelhead flies -or- regional favorites likes those popularized in the Canadian Maritimes. The latter are mostly UK flies tied hairwing style. I would tie up some wolly buggers, muddlers, popcicles and other popular patterns you can find recipes for at first, then move to new patterns that you get an urge to fish once you get the feel for the fish and the river. Of course you could jump right to the Jock Scott -or- learn how to twist up Spey flies but you really don't need to at first.
For gear you should have an 8wt or 9wt rod with a sinktip line first, floater second. Spey rods are really the best way to fish the river but that's another story.
Opt for a fairly long leader, 7-9 ft, and the biggest tippet you can get away with in the conditions - this is typically 10-12# test but often as little as 6-8#.
A sinktip system lets you swap sink rates streamside with no extra spool. This is a big plus because each stretch has a different character in depth, current and bottom features.
Once you get all the gear down you need to read the water to cover those features of the river where a salmon might be sitting. The technique is to swim the fly through these lies at a speed that is offset from the current until you get a grab.
There are also large trout and smallmouth there as well. I hope we can hit it together this spring.