I think most of have been in that situation, I know for sure I have.
My own ISP is at present offering a rather nice discount on their package deals, and by changing the package I have at present, for a very same or similar package, I could perhaps save £100 per year.
But, and its a big but, I have a rolling contract with them, which leaves me with the option of leaving at anytime. With the new deal, they will lock me into an 18 month (no get out) contract. Perhaps its me, but I simply hate being locked into something, that as it regular problems.
There are other ISP's clambering for my business, at possibly even better deals, but again, the devil you know-the devil you don't, is something that I will stick with.