I've been using the Broadxent modem supplied by SE since service was started. My margins were OK, so I didn't have any worries.
I did buy a Westell 6100 on Ebay, however, just for fun.
Lately, I've been having some still-unexplained line problem, with margins dropping low enough to cause the Broadxent to lose sync. For laughs, I tried the Westell and sure enough, results were markedly better. Broadxent loses it at around 6 dB SNR, and Westell hangs in to 4 or even 3.
Both modems report very similar line stats, so I think they are reliable. Westell starts delivering data much sooner after connection is made than does Broadxent. It even runs cooler.
Also, as reported by others, my ping times are better with the Westell, even during periods of good signal strength. BBR reports 12 mS to my first hop with Broadxent, 9 mS with Westell.
In summary, I'm a believer now. I'd suggest picking up a cheapie 6100 on Ebay as a backup to the Broadxent even if your line is strong and clean. And if you have a marginal circuit, it may be your solution.
The Westell seems superior in every way.