We've had insurance (24 hr Petwatch) for a number of years for both our dogs. One of our dogs had knee surgery years ago that was very expensive and we got 70% back from our insurance. The same dog recently went through a serious illness and sadly passed away a couple of weeks ago. We spent well over $3000 in a matter of a couple weeks just in trying to stabilize and diagnose her illness. We got about 70% of that back as well and I just submitted our final claims for reimbursement. I think we've spent 20-30$ a month for her premium for about 7 years so if you add that up and what we have gotten back, we have made out pretty well.
Our other dog has been much healthier and so I don't anticipate getting our money back there (although you never know). However, the peace of mind in knowing that when/if something goes wrong we don't have to worry as much about the financial aspect has been well worth the money.
I can recommend ours pretty highly although you need to pay attention to issues such as whether your pet has any preexisting conditions (won't be covered so good to start early). Also, what the copay policy and lifetime limits are. Our plan pays 70% after a per incident deductible of $100. We then have a lifetime limit of $3000 per covered area (musculoskeletal, cancer, etc.). Also, general check ups and preventative care may not be covered (weren't in our case).
I almost cancelled our policy over the last couple of years, but in the end I am very glad we had it. It made the end of our girl's life a little easier to bear even though it still was very sad.