We purchased a 18v Li-Ion impact driver from Makita 2 years ago, and to date it's been one of the best purchases we've made and is the most useful tool at camp.
Everything takes a lot of abuse at camp, especially the tools that build and maintain the facilities. Our Makita impact has been dropped, thrown, covered in sand and mud and driven a few hundred thousand screws and bolts, but still works like it's new. It's used constantly from minor repairs and fabrications to major builds of new ramps and structures and has yet to disappoint. Sure it's expensive, but it's well worth the investment.
Hope that helps, it's also not a bad idea to do a little research and see what brand/model will best suit your personal needs. Hitachi, Milwaukee (for drills) and higher end DeWalt products are also good bets. Check out
www.coastaltool.com for a good selection of tools including cordless impacts and drills. They usually have pretty good prices and you can get refurbished tools that are still in great condition for a lot less than the same tool brand new. Here is a link to the impact set we purchased and will probably be purchasing a few more of in the near future.
http://www.coastaltool.com/a/makita/btd142hw.htm 
