I'm likely to buy shortly as I have a time-limited budget at work. But were I not under the constraint I'd probably wait. The main upcoming difference is the next generation integrated chipset will use slightly less power and deliver a lot more in terms of graphics without having to resort to the dedicated graphics chip. All the latest DirectX and OpenGL standards will be supported, in hardware, by the dedicated chipset. So you'll see better battery life in practice -- and, perhaps this will finally be the point at which Apple removes the arbitrary limitation on 15-inch Macbook Pros against integrated graphics driving external monitors. In that case you'd have a cooler-running computer in desktop mode as well. I think it would also be realistic for Apple to consider offering a cheaper, longer-battery-life version of the 15 with integrated graphics only, no discrete chip.
The other thing that's always a possibility is more generous standard configurations of SSD and/or RAM capacity. If the next generation maintains the same price points, I'd honestly be surprised if Apple doesn't bump the standard SSD configuration to 512GB in the 15-inch and 256GB in the 13. I suspect they'll still stay at 8GB RAM but you never know.
Increased speed for extreme tasks? Probably not that you'd notice much.