Metacritic
To be honest, there's more good reviews there than I remembered. Still, the other half of the reviews there are going to complain that the control scheme was terrible (i.e. doesn't play like Halo), the vulcan cannon and hovering should be easier (both of which were completely useless), and Naglfar's Pit was long, tedious, and badly designed (a level that can be beat in under a minute if you know how to boost properly).
I agree that Gun Valkyrie was a slightly different problem than shmups in the mainstream. Until you learnt how to boost around properly, it wasn't a lot of fun. By contrast, the 'learning curve' for shmups is short: press the shot button and don't navigate into bullets. But to actually be able to accomplish something takes a lot of time and practice. Most people (and reviewers) are going to: a) die a lot, get frustrated and call the game impossible, and quit; or b) credit-feed through the game, declare victory, and move on.
Admittedly, fighting games have had a huge resurgance, but I just don't see that happening for shmups--even if Cave starts localizing--as much as I would like it. It still might be economical, if the sales for RFA are as high as they seem to be, but they'll never be 'mainstream'.