I bought island property 3 years ago and it is by far the best thing I have ever done. So much so - I made the commute from Bear Island down to work in MA this morning just because I could not for the life of me leave last night it was so beautiful! Yes it was worth getting up at 5:30 this morning - sit on my dock to watch the sun rise (with a cup of coffee) then off for 2.5 hours of drive time. Sadly it's depressing to be at work... but there is next weekend to look forward to!
My first suggestion is if you are going to buy a piece of island property, get a good real estate agent that knows island property and the lake. As mentioned by me a number of times, Nancy Deporter and her husband Doug are the best.
http://www.nancydeporter.com. You will find several others who would recommend them as well. There are others that claim to be specialists in island property but fall well short! This is really important because if you are not familiar with the lake, or some of the rather interesting construction techniques that could be problematic, they are the ones to help with pointing these things out. Plus you can't beat a fellow islander selling island property.
There are plenty of businesses that provide support for islanders for everything you can think of. Yes it can get a little pricy to do things such as major construction jobs, but if you can dream it up there are folks that can do it for you if you aren't able or adventurous enough to try it yourself. So really the price to buy may be cheaper, but that can be offset by hiring somebody to do stuff for you. It is helpful to be handy that's for sure.
Far as everything else, there is nothing you cannot get to by boat - there are grocery stores, hardware stores, and everything in between that are accessible by a short walk from most town docks. Most islands have full services such as electricity, some with cable and high speed internet. Depending on your situation mainland "logistics" are a consideration. You may or may not have a good place close by to launch your boat, if you choose to trailer up every visit, or you can either rent a slip, rack your boat at a marina, or buy a slip. That can add some overall cost to your annual cost of ownership.
I suppose it comes down to what you are after and if the island "lifestyle" is what you crave. From a personal stand point I wanted a place to completely disconnect and have that feeling of being isolated from everything. I don't mind the adventure, love how quiet it is, my neighbors are awesome folks, and it's a fantastic place to have what I consider to be the ultimate getaway. Quite frankly at this point, no matter if I had millions to spend on a mainland property I would not give up being on an island, no way.
Good luck in your search, there are some nice places for sale right now.