I'm currently reading the third book in this series, so my review will probably end up being pretty general over what I've read so far.
I think the mark of a good series is when you feel like they are all one big book, when everything flows and fits together properly. I hate when one book is really good and then the next one stinks.
The mark of a good AUTHOR is when the chemistry with the characters is real. I don't necessarily mean just romantic chemistry. The main character, Rose, is best friends with a vampire princess, Lissa. Their friendship is pretty complicated, and all the feelings between them are just so right. Rose is Lissa's protector, and they also share a mental bond. The tension those things cause between them is very realistic and I just love how the author portrays them.
The romantic relationships are also great. Rose becomes involved with another protector (or dhampir, as they're called in the series), but their relationship is complicated also-forbidden actually, because they are both set to be Lissa's protectors when Lissa and Rose graduate. Their chemistry is so thick and vibrant, you can actually feel it when you read. Heck, Dimitri's character itself has enough chemistry to visibly spark from the pages.
This is easily one of my favorite YA series that is still in progress, but all six books are scheduled to be published by the end of 2010, so at least there won't be a long wait to find out how everything comes together in the end!