I actually have a couple of friends (big Buffy fans) who regularly discuss how much they dislike her as an actress. Which always surprises me, because I enjoy her work.

I voted "neither, cuz I like her on both," but that's really not exactly true. I think Alyson did a great job with Willow when the writing was good. I loved her in early seasons of BtVS, I loved her as "dark witch Willow." But when it became "Willow knows all about darkest magicks and is the pseudo wise shaman of the group," I hated her. I also hated her schmoopiness with Tara and subsequent relationship with Kennedy. (Not that I hated the character once she became a lesbian, I just feel like the writers weren't exactly sure how to handle it at times, and Alyson's portrayal of Willow suffered for it. I feel as if the Kennedy relationship was forced [what did Willow see in Kennedy?], and she and Tara were totally fine when they weren't ridiculously schmoop-tastic.)

Likewise on HIMYM, I think Alyson is fine when the character of Lily is consistent. She's a bit of a bitch at times, and a horndog, and is a compulsive spender, all of which makes her unlikable at times. But she's also a loving wife and friend, which Aly pulls off well enough for me. When I don't like her, however, is when her character seems to do something completely out of left field that doesn't make sense according to what we've been shown of Lily thus far. But if you look at her onscreen chemistry with Jason Segel, she's on. Even with NPH, she's on. (Consider the episode when Lily moves in with Barney, or when she finds out he loves Robin. I don't think those scenes between two characters—who ultimately have no real reason to like one another—would work if both actors weren't on their game.)

So in conclusion, while there are times when I actively dislike the characters of Willow and Lily, and/or how Alyson portrays either character, it ultimately comes down to the writing not being consistent enough for my liking.