Sabres fan here, in peace.
Darcy was a solid GM that spanned 3/4 different ownership groups (the cheating Rigas family, NHL, Golisano, Pegula). He inherited a decent team with Hasek in net and managed to add a few pieces that got us to the Cup finals.
Under the NHL ownership and the Golisano ownership, he was basically told not to lose money and he did so. He's often pilloried in Buffalo due to "his handling" of the failed contract negotiations of Mike Peca, Chris Drury and Danny Briere. More astute Sabres fans know that he had higher powers more or less calling the shots and he did the best he could under the given restrictions. I think if you got Darcy in an honest and drunken moment, he'd tell you in his heart of hearts that he wanted to take Edmonton's 4 first round picks for the Vanek offer sheet, but knew that since Golisano had said that he needed to at minimum break even every year.......he knew he needed Vanek to make the playoffs and couldn't rebuild right there and then.
Basically, what you're getting in Regier:
PRO: Very solid drafter, bordering on elite. Has a tendency to "win" most trades that he's not forced to make....and often it's not really debatable. Patient.
CONS: Reactive, not proactive....and slow to react. Hesitant to trade away talent that he drafted.
MIDDLE-OF-THE-ROAD: Free agency history is mixed. Does VERY well at finding the lower-cost FAs that fill a roster. Not so hot at the higher-profile guys