If you value the friendship more than a possible romance then yes, its a bad idea. But if you both can still be close friends even if the romance doesn't work out then there's nothing really to lose, but in most cases the friendship just isn't the same. Coming from personal experience and observation :P

well in a way i am married to one of my best friend
and one of the only friend i got left so i don't see why it's not a good idea

just have to both set rules that whatever happens the friendship has to stay on and never stop i think

Hey, what do you mean "in a way" you're married?!?! Is there something you forgot to tell me? *throws popcorn from across the room*

Now that that's taken care of...
In my opinion it really depends on the each person, the level of maturity and of friendship they both share. Also if both of you are having the same level of feelings for each other, obviously ^.^ To me the biggest risk is (as several others have said) that if the relationship sours badly, you risk loosing both your best friend and the relationship at the same time. If both of you are adult enough to recognize down the line that it may be better to stay friends rather than lovers, it could be a big help so that you don't wind up hating each other in case something does happen. I think the best advice I could give would be to sit down together and talk as objectively as possible. Talk about how you feel, but try not to be too overwhelmed with making the "date or don't date" decision. That said, if you both feel strongly for each other, beyond friendship, you never know what might happen!

/ramble off ^.^

I can't tell you this isn't a good idea.

Honestly I was best friends with my best friend for three years, now he's my fiancee and I've never been happier. Funny how friends turn out to be soul mates. sometimes knowing someone before getting to be romantic with them works wonders because you know each other before all the other stuff.

yes and no it depends on both peoples feelings and if they're mutual
also there is a risk of both ending up not liking each other as much, or the exact opposite it could be great
but only if feelings are strong enough and if they're mutual
thats most important

as to your situation my take is if your hearts in it then go for it

it really depends...
and if it doesnt work out in the end you risk to lose both the relationship AND your best friend :T

I value friendship over relationship. But I have dated a few of my friends. As long as the two of you can still remain friends afterwords, there should be no problem.

Go for it.