To be honest its not that I feel that watching the anime spoils the vn. Rather that it really depends on how the anime was to the person watching the anime that will motivate them to play the vn or not.
Well, its more of a story of what happened with me and my friend when we watched the Tsukihime anime. It was last year or maybe two years ago that my friend talked to me about the tsukihime vn. I had already played through it and.... he wanted my honest review of the VN. To cut a long story short, he watched the anime before playing the vn and was really disappointed in the anime(me too). I told him that the vn was great but it was a hard to persuade someone that had already seen something that gave him a mindset that shows that this game sucks ass. Well, after much coaxing and having him read the story for a few minutes, I managed to get him to actually take an interest and play the whole VN. Well, I think he has finished reading all the routes by now. But, to be honest it would have been a whole lot easier to persuade him if he had played the VN before watching the anime.
So, my opinion after that was that watching the anime gives you an idea of what kind of outcome you can expect from the story. The anime might lack depth and ...enjoyment(hehehe). But, the whole anime gives you an outline of what you are putting yourself into reading a dull book about some wimpy hero and some overly stressed out vampire with a problem. Don't get me wrong, I love Tsukihime as a VN. Its just the anime that I couldnt stand.
BTW, I also read Steins;Gate, Chaos;head, and fate/ stay night after watching the anime and honestly felt that they were great and made me want to rewatch the anime again to reconfirm some things in the VN with the anime. But, yeah some times that just doesnt work with all animes. (especially if it was bad)
So, for the question of whether I would play the VN it after I had watched the anime. Yes, I would. But, from experience I can see some people wouldn't touch it if they disliked the anime.
P.S. I still dont like the Tsukihime anime. Wish it had been made like the VN. -sigh- oh well, there are others awaiting.