Hellsing Ultimate is not a followup to the TV series, it is a stand alone OVA that follows the manga storyline very closely. When I watched it through the first time last night, I had issues with the art style, mainly Alucard. After watching it again, it really isn't an issue anymore and I got use to it. The artwork between the TV series and the OVA is quite different. What Gonzo did was pretty amazing visually but it does not follow the art style of the Hellsing manga, which is also quite good. If you were one of those that read the manga, then saw the anime and was disappointed, you SHOULD watch the OVA. Even though Hellsing the TV series was very good visually, Gonzo changed a lot of aspect of the original story and had a really fucked up ending.
for those who want to read the manga
http://groups.msn.com/ATU-BOTI/miscmangasamples.msnw