I've used this box for about 2 weeks now, both QAM and OTA. I previously owned an LG 3510. I prefer the Samsung for a couple of reasons:
1) More stable QAM decoding;
At $242 (compared to the LG at $399), the Samsung really only has one glaring weakness: the inability to directly select sub channels. Aside from that, my only other real complaint is that it has a very annoying tendency to crash (unit powers down than display says booting) when searching for QAM channels above 111 (it is fine once it reboots). Otherwise its performance is comparable to the LG. It changes channels nearly as fast or faster (provided you've memorized them and discarded unused channels; otherwise, it's VERY slow), and the menus, while not as attractive, are functional (neither has a digital signal strength meter). There's no discernable difference in the picture between the two units, and the SIR-T451 was able to pick up OTA channels from as far away as 40 miles with an amplified indoor antenna. Both seemed to do a good job resolving OTA signals. However, in QAM mode; the LG would occasionally drop the signal (it just sort of glitched for a brief second every few minutes on almost every channel), while the Samsung really locks in and I rarely see even a hiccup. Oh, and the LG would erase all the channels in memory for QAM when doing an OTA search, then I'd have to remap QAM when switching back to the cable input. The Samsung retains the QAM and OTA channels in memory even when I switch input types.
The T451 isn't perfect, but, for the price, its ability to decode both OTA and QAM makes it an attractive option. Since TWC NEO is running several QAM channels unscrambled on their system, this box plus my Panasonic 47 HD-ready TV has put me in HDTV land for under $1500. So, I'm happy.
Oh, and as an aside: My neighbor (he wrote the review above!) is waiting on an LG 4200, so I'm sure we'll be able to do some side by sides with these two units in the not-too-distant future. At $279, the 4200 is not much more expensive than the 451, so I'll be anxious to see if it is better than its sibling the 3510.