The main thing that draft 11n brings to the table is higher throughput. After all, the official name of the IEEE Task Group n is "Standard for Enhancements for Higher Throughput". What is higher? Figures 1 and 2 show a throughput vs. path loss (signal level) comparison of four Linksys products that use Broadcom wireless chipsets.
The WRT54G [reviewed] is Linksys' ubiquitous standard 802.11g router. The WRT54GS [reviewed] is 802.11g enhanced with Broadcom's "Afterburner" non-standard 802.11g throughput-enhancement technology. The WRT160N [reviewed] is a two-antenna, 2 Transmit, 2 Receive (2T2R) entry-level draft 11n router. And the WRT600N [reviewed] is Linksys' top-of-the-line, dual-band dual-radio three-antenna draft 11n router.
Figure 1: Downlink throughput comparison - Linksys
The gist of Figure 1 is that standard 802.11g will deliver 20-25 Mbps, while Broadcom's speed enhancements will boost that about 10 Mbps. Moving up to a relatively inexpensive entry-level draft 11n router will about double the "GS" throughput to around 60 Mbps. But making the final step up to a top-of-the line draft 11n router provides a boost similar to the G to "GS" step, i.e. ~10 - 15 Mbps.
One important factor is that, aside from standard 802.11g products, you get these best-case results only with matching wireless adapters. "Matching" means a wireless adapter that uses at least a chipset from the same manufacturer and product family. 802.11g throughput enhancement techniques used by different chipmakers while similar, are proprietary and don't usually interoperate. And, until the "draft" drops from in front of "802.11n", you can't be sure that a router and client are operating at optimum performance.
To illustrate the point, Figure 2 compares D-Link products that use Atheros wireless chipsets. The WBR-2310 [reviewed] actually uses an Atheros "Super-G" (its proprietary throughput enhancement technology) chipset, but defaults to the "Turbo" (channel bonding) part of Super G turned off. The DGL-4300 [reviewed] is also Super-G based, but defaults to having "Turbo" enabled. The DIR-625 [reviewed] is an entry-level two-antenna draft 11n router and the DIR-655 [reviewed] is D-Link's current top-of-the-line three antenna draft 11n router.
You can once again, see a similar pattern of throughput improvement as you move from standard 11g, to "enhanced" 11g, to two-antenna draft 11n, to three-antenna draft 11n.
Figure 2: Downlink throughput comparison - D-Link
Figures 3 and 4 are the uplink equivalents of Figures 1 and 2 for the Linksys and D-Link products, respectively. Look kind of different than the downlink, don't they?
Figure 3: Uplink throughput comparison - Linksys
I included these to show how products can behave very differently between up and downlink performance. Some applications, such as video streaming primarily require a fast downlink connection. But others, such as general file sharing and transfer, require high bandwidth in both directions.
Figure 4: Uplink throughput comparison - D-Link
You might think that draft 11n products have longer range, and that's just what manufacturers would like you to believe. But if you look carefully at their claims, they usually refer to "coverage", with dimensionless charts that imply that you'll be able to connect at a longer distance, but don't provide any numbers to back up the implications.
A look at the charts above shows a more accurate comparison of the "range" performance of the four Wi-Fi technologies. If draft 11n products really provided longer range, their curves would end at a higher path loss value (lower signal level). But the data shows that they actually stay connected to similar signal levels. And, in some cases, the draft 11n products actually disconnect earlier than 11g and "enhanced" 11g products. You can find many more examples by running your own plots using the Wireless Chart tools.