If you disliked fate UBW b/c of the characters then you may like Zero since the characters are some of the best in anime. but if you just hated the whole thing then don't. The focus of Fate Zero is more on the servants than the masters, and I liked that more since the servants are more interesting and unique than Rin and Shirou.
I think with romance anime, the enjoyment of certain shows will depend on how much you enjoy a show. That is at least is what I have noticed in myself and those I have talked to. Or if you personally connect to the MC's situation. Those factors may boost your enjoyment/investment in a show with meh plot and themes. I could be completely wrong, but that is just what I have noticed.