This weapon is very impressive for a Boomkin. Testing on a target dummy I had an average increase of about 700 dps. In 25 man raid I assume it would be even more. On average in ICC 25 man hardmode content the Val'kyr damage accounts for between 2%-6% of my overall damage each fight.
The Val'kyr hits for 3150 damage per tic usually per second, sometimes it does a double hit (3150x2). This lasts for 30 seconds. The damage does not scale with sp, it is the same damage on a target dummy and in 25 man H raid fully buffed.
From what I have observed so far, the more aoe damage you do the greater chance for the Val'kyr to proc. So might not perform the best on a single target boss. (This is not confirmed, just my observation).