I just installed the BestSkinsEver on my iPhone 3G and my gfs iPhone v1. I have to say I am very impressed. I have an InvisibleShield on my 80GB iPod and it definitely has been great for me, so I wanted to cover our iPhones too. I decided on the BSE due to the MUCH lower price for it (I got 2 full-body 3G skins and 1 V1 skin for less than 1 InvisibleShield).
Since I have the IS to compare to, I have to say I notice zero difference between the two skins. They are the same, as far as I can tell.
I also was worried about the effect the skin would have on the iPhone but I must say I am very happy with it. It does not affect the viewing at all, IMO. In face, I think it feels better. Fingerprints dont seem to stick to the screen as much as it did without the skin. If anyone wants pics, lemme know. I have tested the skin in very bright sunlight, darkness, etc and there is not only no difference in the image quality of the face, but I swear it does look better!
I also have the
Incase Leather Sleeve for iPhone 3G (Black) (yes, I like to take care of my iPhone)

. The skin and case work very well together. Really the case is just so I can clip it to my pocket, and the skin does the extra job of keeping the whole thing scratch-resistant and pretty.
The application for the BSE skin took a total of 45 minutes for both phones, and that was just because I was VERY particular about getting it as perfectly placed as possible. You are supposed to let it sit, unpowered, for 24 hours but I didnt do that. I let it sit for about 6 hours and turned it on. Looked good, no issues. I did do it at night though, so it would have the night to settle itself. I took BSEs advice and used some baby shampoo (Johnsons no more tears!

) in a small shallow bowl of water.
Overall, I have to say I am very content with the BSE. IMO, save the cash from the IS and get the BSE.