Drew Coleman is a guy who gets a lot of criticism. Some of it is deserved. I felt like he really improved his game in 2010, though. I had not really been able to find numbers to back up what I saw until I came across a Football Outsiders study of defenders who made tackles on plays that prevented receptions from becoming successful.
Stops: The total number of plays by a defensive player that prevent a successful play by the offense, defined as 45% of needed yards on first down, 60% of needed yards on second down, and 100% of needed yards on third or fourth down. In general, "plays" refers to tackles, passes defensed, fumbles forced, or interceptions. The exception is when discussing pass defense data from the FO game charting project, in which case "plays" refers to all charted passes with the given player as the listed defender.
Coleman gave up a fair share of completions, which resulted in 22 pass tackles according to the report, but he also made 8 stops on those plays, which FO says makes him a top ten player in this category.
This is not a perfect system. Coleman might have been so burned on some plays that he might not have been able to make the tackle when he gave up a completion. That seems to be the case to a degree. Pro Football Focus says Coleman allowed 38 completions in 2010.
Nobody is claiming that Coleman is a top notch nickel, though. It is just that the perception he is terrible because he allows completions is not always true. There were a number of times completions did not hurt the team because he made a tackle to limit the damage. He probably is not as bad as people think.