Yes and I can post hundreds of links to happy customers. The SB6121 was one of the most popular and well regarded modems last year. The newer SB6141 would be a better bet for someone buying a modem now as it supports 8 bonded download channels vs 4 for the SB6121. As has been said by others in this thread the thread starter needs a Docsis 3 modem for his 55Mbps internet package. Docsis 2 cannot support that speed.
You can see the Amazon rating for SB6121 at 4.5 stars from 1786 reviews...................
Also see this poll for which modems are popular on Comcast the SB6121 is second only to its close relative the SB6120