Can faster be funner? Yahoo! thinks so.
I know, I don’t love it either. It kind of makes the peevologist in me cringe. I want it to say Faster is more fun. But I mean, really, blech. What kind of campaign slogan is that? Definitely not funner.
Fun has conventionally been said to be a noun, so that you can’t make these comparative or superlative forms from it. . . . What we’re seeing here is language evolution in action. So the straightforward answer to your question is: yes, you can use these forms, and people are doing so increasingly often. But if your question was really asking whether it is acceptable to use them in all circumstances, then I have to say firmly that, no, it isn’t, not yet anyway. They are definitely informal and they should still be avoided when speaking or writing standard or formal English.
Grammar Girl offers a similar analysis, calling the use of funner an example of “language in flux” and cites the 2008 “funnest iPod ever” campaign (another example of where strictly toeing the line—the most fun iPod ever—probably just wouldn’t have worked as a campaign).
In the end, I’ve come to believe that there is a “fun” continuum. On one end you’ve got “fun,” the noun, and everyone is happy to cluster around and be associated with it. That’s the standard usage. Then, if you move on to “fun,” the adjective, you’ve got a smaller but still significant group of people who will give their approval. That makes “fun” as an adjective informal usage. And then as you move on down the continuum you’ve got a much smaller group of people who are willing to grab “funner” and “funnest” by the shoulders and give them a big welcoming hug.