Not only are most of the the pre-N and draft-N routers on the shelves today going to be incompatible with the final spec, they're incompatible with 802.11b/g products (and with each other). Three chipsets are on the market today that are being used in these N routers: Airgo, Broadcom and Marvell. In tests, Airgo received the highest speeds, but completely obliterated any 802.11b/g router in the vicinity. Broadcom and Marvell wasn't as bad, but they still interfered with legacy gear.
What this means is if you're using both 802.11n and 802.11b/g in your home, all your b/g gear will have loads of problems when your newer wireless is in use. And just forget about using this in an apartment complex, where many wireless routers are close together—your neighbors will be pissed. The problem could be solved if you were to use a high gain antenna with your N gear, but nobody really uses it until they need range, but if you need range that means nobody is close enough to you anyway.
So once again, here's another compelling reason to wait until N is done before upgrading your wireless networking. But, if none of these problems affect you—live close to others, buying N gear from different vendors, need to use both N and B/G simultaneously—then pre-N could be alright.