Marlon Byrd is, by definition, average. His career wOBA is .332, and most projections expect that to continue. His career UZR/150 in CF is 0.0, with good range (5.5) and a poor arm (-5.0). Dave "The Master" Cameron over a Fangraphs examines Byrd now, and David Golebiewski reviews Byrd's history and fantasy impact.
Considering our options (and budgetary constraints), Byrd was about the best option out there. He pushes Kosuke back to RF (which is good) and came pretty cheap at ~$5M per year (which is good). He is aging and should be certifiably "old" (in athletic terms) at the beginning of his final contract year, but he's not here to be our savior, so that's acceptable. What would be unacceptable would be a fan base expecting 20 to 25 home runs 10 to 15 stolen bases, and then booing when he doesn't produce that.
Ultimately, though, I applaud Jim Hendry for making this acquisition, though not the route through which it came (the Bradley-Silva swap).
Sadly, the Cubs, version 2010, enter the season with much less talent (and thereby potential) than the Cubs, version 2009.