Instagram is finally available, whether you like it or not. I doubt I'll use it, and I'm sure many Android users dislike the idea as well, but personally I wish Instagram huge success on Android. Why? Because Instagram is one of those apps (like Infinity Blade or Instapaper) who *refused* to come to Android for a very long time - for whatever reason.
If Instagram were to fail on Android, it would send the others a message that Android is not worth it, and the Instagram company will start justifying how this is why they never wanted to be on Android in the first place. But if it's tremendously successful when being ported to Android, the others will notice as well, and it will only help Android be taken a lot more seriously by developers who are "iOS-only" right now.
The Instagram team said that they are signing-up 2000 new users per minute, or about 3 million per day. Even if that level of adoption won't be sustained for a very long time, it should still give Instagram at least 15-20 million users in a matter of weeks. The iOS version took a lot more than that to reach 30 million users, but obviously Android benefits from the popularity and marketing it had coming from iOS.
But this still shows that delaying porting to Android for so long was a big mistake for Instagram, and if they double their userbase in a matter of weeks, and then keeps growing much faster still, because of the network effects between so many people, then a lot more "iOS-only" companies will take notice.
One thing I wish they (and all developers) took more seriously (and Instagram doesn't do it much either) is that they used the ICS style guide, rather than port their app completely with the iOS design. Give Android the respect it deserves. It has the same userbase as iOS now, and there's no excuse for not taking the time to use the design guidelines on this platform. But once Android 4.0 gets at least a 50% market share (which hopefully will happen by the end of this year) from the whole Android market share, I think we'll start seeing a lot more developers taking the Android style guide seriously.