What do you like best?
I like that they offer a good amount of free vectors, unlike a lot of other sites that try to force the premium/paid options on you, and they're good quality!
What do you dislike?
The downside to having such a large library is there tend to be some duplicates, or very similar artwork, but as long as you're willing to do a little searching it's not too much of an issue.
What business problems are you solving with the product? What benefits have you realized?
Vecteezy provides a quick and simple source of vectors for a lot of different projects, saving time and money.