Instance leveling can be faster but it's not necessarily the best way to do it. Yeah you get better gear and what not but a lot of times someone who is questing in groups can level up faster. It all depends. Plus, doing the same instance over and over gets old really fast, I leveled a character from 60 to 70 and it's pretty damn redundant. Another problem with instance leveling is that you lose out on a lot of rep, I mean I know it'll probably be like TBC where you get rep for kills in an instance but the quests still give you more. With all the knowledge base that I have right now. In addition to this you lose out on story line for some of the quests. While some people may not have any problems with this, I'm just providing some counter points to instance leveling. But on the other hand, you can get more gold by going back and completing the skipped quests, but that usually takes a lot of drive due to the fact you're not getting that much of pay off like the pay off experience usually holds.
I leveled my rogue 60 to 70 based on quests alone because they excited me at the time (that's what she said) because they were new and they weren't the same old grind and repeat that they were in original WoW. But I leveled my warrior 64ish - 70 all with instances because I had already done the quests and they were less appealing. The only problem I had with that was I still don't have all the flight paths in Outland on that guy and it was extremely boring after the first couple runs because there was no diversity it was the same setting for like 10 runs or ~3 levels. However, it was something relatively new because I didn't run instances on my rogue, but after the new instance aspect ended it was quickly boring. Instance leveling is pretty successful, sans the boredom aspect, and it's still too early to tell the difficulties of the instances because it's just the beta, they'll probably make some tweaks to the difficulties of the dungeons prior to the release.
I'm not trying to shoot down anyone's ideas here or anything like that I'm just making a statement from my perspective. Personally, I probably won't instance level with whoever I level to 80 first because I want to experience all there is to Northrend in the form of quests and I want to get the storyline of those quests. Plus, there's nothing more exciting then seeing thousands of players all in one zone trying to quest. It also gives an added break from the raiding scene. I don't want to do the same thing I do in raids from 70 to 80 because then it just burns me out faster. I need a break from it.
However, with all that said, I'm not trying to weave an argument here or anything. I'm simply telling you about my ideals. I will probably instance level one of my characters because despite all of the redundancy it does level pretty fast, esp. if you're rested.