Hot Grips is what I chose. They had the best reviews that I could find for heat output and longevity. They were pretty easy to install but a little time consuming due to the wire routing. Here's a link to their site:
HOT GRIPS-heated grips, hand warmers, grip warmers
I got mine with the switch house and then picked up a Rubber Toggle Boot to protect the switch from the elements and make it look cleaner.
I've rode in 24 degree temps with Held Warm and Dry gloves and it was very effective and kept me very comfy. Anything below 40 degree's and High setting is needed. I have no doubt these would work fine in the single digits but you will notice they feel cold whenever you take your hand off the grip at speed but once you put your hand back you'll feel that layer of heat building back up.
We had -17 degree's where I live for the last few weeks, luckily the low has gone back up to +15 degreee's. And everyone always thinks that all of CA is beaches and all of NV is roasting deserts. Go north and it's mountains and cold winters.
Oh just to mention if you order from Hot Grips add the Epoxy and fuse relay to the order as it doesn't come with it. You can find the epoxy at some stores but it might be a pain and it's nice to have everything when you need for it. It's an epoxy that breaks down at a certain temp by insulating the grips so you can remove them without destroying them if you choose. Also they suggest you don't use heated grips with Throttle locks as the grips expands. I don't think that it would be an issue to use them though if your doing a long trip and you don't need to turn the grips on.
Here's a couple pics of them installed on my Honda:
You can see the switch housing to the right of my high beams which is pretty clean. Requires no modifications to the bike as it just goes around the bars. Might be nice to just buy one of these if you need a switch for something and don't want to drill the fairing. They have 3 different type at the bottom of the page for different switch configurations:
Accessories You can always buy just the house if you have a switch already.