I tested the NIC's performance using NetIQ's Chariot 4.1 and a selection of 11a and 11b access points. For 11b throughput tests, I used Cisco Systems' 1200 series access point. I performed 100 iterations of 1-MB TCP file transfers (filesendl and filercvl) and was surprised by the transmit results of 6.3 Mbps and receive results of 6.1 Mbps. That's faster than Cisco's Aironet 350 cards, which we use as the performance standards for 11b NICs. Additionally, I found no significant degradation in throughput with WEP (Wired Equivalent Privacy) enabled.
To evaluate range, I installed the AP above a dropped ceiling in our test facility and ran a continuous ping test until the pings timed out. Even with multiple walls and steel-and-glass doors, the combo card working in 802.11b mode achieved a maximum range of about 170 feet--again comparable to that of the Cisco Aironet 350. Based on these tests, it looks like you won't have to sacrifice 11b performance with these NICs.
I used the Netgear HE102 802.11a AP, based on an earlier Atheros reference design, for range and performance tests of the card in the 802.11a-only mode. With the same setup used for 11b testing, the average transmit and receive throughput was approximately 22 Mbps. Again, performance degradation with WEP enabled was nominal. These results are slightly better than that of the Netgear HA501 802.11a card I tested in the lab. The maximum 11a range of the combo card in our environment was 115 feet. Atheros said range should improve once APs based on the AR5001X chipset are introduced, a development likely to occur by year's end.
802.11a and 11b access in a single NIC.
Good range and throughput for both modes.
Slow roaming handoffs between modes.
Large antenna footprint.
Confusing configuration options.