I was with Hibernian/Aviva for two years and had no problems with them, though I never had to claim. A few people on here have talked about claiming form them and I don't recall any negative experiences being mentioned. Use the search function to have a look for yourself.
As greenie says, use the Health Insurance Authority website to do a comparative analysis, it might bring up other lower cost alternatives.
Aviva covers pretty much the same hospitals as VHI, but if there's any particular consultant you're currently using or are likely to in the future, have a look on Aviva's website to see if Aviva covers them. I haven't heard anything bad about their consultant coverage, but no harm checking for peace of mind.