After finishing it, I pretty much stand by my initial statement: the series falls in-between Charlie’s Angels and Buffy/Angel.
Part of this reason for this is that it doesn’t seem to me that the writing is particularly good. They seemed to try to set up a lot of little plot and character threads — especially in the later seasons — and then try to wrap all of them up in the last few episodes. But there were always too many of them, and so most of the time everything just seemed rushed and not properly closed. The worst, for me, was the Season 6 finale and the start of Season 7, but even the last episode ran into this. Sure, showing Leo and Piper as grandparents was nice, but it was too short to work as a finale-defining moment for them and didn’t fit well enough into the plot — ie it didn’t do anything wrt the plot — to come across as anything other than that, and they had to quickly resolve all of the plot threads, including getting Billie to kill Christy, which then had little impact because it was both so sudden and they didn’t follow up on Billie’s obvious grief and problems from having to do that. Which they couldn’t do because the episode and series was almost over.
This, I think, is also driven by their attempts to develop the supernatural plots more than they started out doing. In the early seasons, for the most part the supernatural was the backdrop against which the personal problems of the witches were displayed and developed, but later the details of demons, warlocks, the Elders, the Avatars, and so on and so forth were explored more. While this made the show deeper, they also did indeed keep the personal problems as well, leaving all of those various issues in need of resolution, with none really being more important to the series than the others. This made for very crowded seasons.
That being said, I don’t think they could have kept that focus on the personal issues of the Charmed Ones because even with the show not having that as the sole focus their personal problems wore thin. The problem here was that they didn’t really turn those problems into personality traits, showing how them having those problems came critically from who they are, and then using those traits as plot points. Instead, they kept revisiting the problems, and so they never really evolved all that much in the series. Your mileage may vary on this point, but to me it really seemed like they kept doing the same problems over and over again.
I think the problem with Billie suffered from this repetitiveness as well. At the start, you had Phoebe in love with being a witch, Piper wanting to be normal, and Prue kinda in between. Then Paige replaced Prue and you had Paige excited about her powers, Piper still wanting to be normal, and Phoebe somewhere in between. At the point where Billie entered, she came in with the same annoying “I love my powers and love kicking demon butt!” attitude, but while Paige was trying to rein her in, it was really the same story over again — and the same lessons — in someone who wasn’t a Charmed One at all. We’d seen it all before and Billie didn’t tie as tightly to the sisters and so we didn’t even have that three-way conflict to balance it out.
The worst part, though, was all of the times that the problems in the episodes were created by the sisters abusing magic in a way that they ought to have known would cause problems because it did all the other times. Most of the time, it was their own stupidity that got them in trouble, and less the amazingly intelligent machinations of their enemies. So many problems could have been avoided by the sisters talking to each other, not casting dangerous magic, or admitting that they had when they did. It got annoying over time.
That being said, it’s definitely watchable. The plots are interesting enough to give a focus to the episodes, the characters are interesting enough to not bore, and are all pretty easy on the eyes. As I said, between Charlie’s Angels and Buffy/Angel.