Overall, Hellsing is actually a pretty bad and horribly overrated series. But that's not to say it didn't have any redeeming factors whatsoever.
It started off pretty well, the storyline was somewhat intriguing for the first half. But after a while, as the series began coming to a close, I felt like shouting "What the hell is happening?!" And then there's the absolute WTF abrupt ending that sort of tries to answer some of the questions very briefly, and then just... stops. The series doesn't 'end', it simply 'stops'.
There are only two reasons I can think of to watch Hellsing: the awesome animation and Alucard (even if he did get on my nerves after a while with his constant cackling, but that might be because I watched it dubbed.) Oh, and the line "Makes me feel like a bite to drink." Classic.
The animation was incredible, overall. It was very detailed, sharp and imaginative. I especially loved the blood-red night sky, very effective. And Alucard is a perfect example of what a vampire should be like. *insert witty insult to Edward Cullen here*
Other than Alucard, though, the characters are pretty damn boring. About half through the series I realised I really didn't care at all what happened to them. None of them are all the memorable, seeing as we don't ever find anything out about them! The most annoying thing in terms of characters was what I assumed to be the main antagonist, Incognito. He just randomly appeared in the last few episodes with little explanation, did practically nothing and then got impaled by Alucard.
That's about all I can say about this series, it really gave little else to review on. Really not worth the time watching.
PS I haven't watched Hellsing Ultimate, or read the manga, which I've heard is a vast improvement, so I haven't become totally disinterested in the Hellsing franchise.