On the fence.
My personal experiences: Lukewarm
I went from an iPad1 3G to an iPad 4 Wifi. I could have bought all the other iPads as they were released, by selling the older model. I think I spent more total to get both a 1 and 4 versus that method.
I never felt that the iPad1 was a slow device. The 1GB RAM is very nice. I wish the 1 had 512MB instead of 256MB. No more reloading tabs or random Safari crashes. Apps load faster(slightly). I don't know how iOS5 support will be after they release 7.0. I suppose that is a reason to upgrade.
I don't read too many PDFs, play Games, Use Garageband and etc. I assume thats where I would see a big difference. I use my iPad1 for was showing photos/videos to clients, reading internet articles, Flipboard, writing, podcast recording and Netflix. Any heavy creative stuff, I use my computer.
I did edit a short film on my iPad. Kinda cool. I'll stick to my NLEs but nice to know a tablet without full PC features can accomplish such a task. Cameras are great but I have dedicated tools for that purpose.
The iPad 1's screen was never horrible for photos and movies. Side-by-side, there is a difference but not enough for me to think I'm missing out, ignoring the resolution which is great for text. I prefer a paper book.
Like someone else said. Upgrade every other generation. I waited 3 to say 'yes'. My iPhone4 was my first smartphone. Still use it and felt it was the right version for the iPhone. iPad 4 seems to way too. It's the right retina device like the iPad 2 was what the iPad 1 should have been. I do feel this is a temporary device. The 5 might be a noticeable change.
Cheers