I absolutely loved this series. It's strongest points are world-building (which even it's detractors acknowledge is some of the best in fanfic), plotting, it's varied memorable and well-developed characters, and good, realistic use of moral ambiguity.
The main fault line between lovers and haters of this series is their view of the characters, especially the main character. Alexandra is definitely a case of love or hate. Many readers find her a fascinating, relatable and sympathetic anti-hero, while to others she's an annoying brat with a tragic backstory. I fall in the former category, those who find themselves in the latter will probably not share my enjoyment of the series. This is also the case with almost all of the main characters. Although many of the side characters are likable in a straightforward way, the characters important to the plot are almost all dark, troubled, prickly, and above all mysterious. Aside from Alexandra herself no character's true motivations even begin to become clear until book 4, and many are still very murky and the amount of emotional dysfunction among the main cast is evangelion-esque. It's a case of reader preference whether to view this as a positive or negative.
Besides that, I'd say the main weakness of the series is the lack of a developed core group. Alexandra has a varied group of friends, but they are rarely actual participants in her adventures, so the team dynamic that was such a big part of canon harry potter is missing. This is partly because Alex has a more solitary personality and partly because she is skilled enough that she doesn't need the help.
Another thing to note is that the series improves dramatically after the first installment. 'Thorn Circle' is well written, but Alexandra is at her most unlikable (even many readers who like Alex didn't in the first book), and the plot is largely a rehash of the first Harry Potter book. From the second book on the character, the series and the author come into their own: the writing gets even better, Alexandra becomes more likable through organic character development, the plot (one of the main draws of the series) picks up and the more interesting secondary characters get introduced.