N50 is most definitely not the only thing to look at. How you should asses it basically depends on what you want to do with the assembly.
You could check out this paper recently submitted to the Arxiv
"Assemblathon 2: evaluating de novo methods of genome assembly in three vertebrate species"
Keith R. Bradnam (1), Joseph N. Fass (1), Anton Alexandrov (36), Paul Baranay (2), Michael Bechner (39), İnanç Birol (33), Sébastien Boisvert10, (11), Jarrod A. Chapman (20), Guillaume Chapuis (7,9), Rayan Chikhi (7,9), Hamidreza Chitsaz (6), Wen-Chi Chou (14,16), Jacques Corbeil (10,13), Cristian Del Fabbro (17), T. Roderick Docking (33), Richard Durbin (34), Dent Earl (40), Scott Emrich (3), Pavel Fedotov (36), Nuno A. Fonseca (30,35), Ganeshkumar Ganapathy (38), Richard A. Gibbs (32), Sante Gnerre (22), Élénie Godzaridis (11), Steve Goldstein (39), Matthias Haimel (30), Giles Hall (22), David Haussler (40), Joseph B. Hiatt (41), Isaac Y. Ho (20), Jason Howard (38), Martin Hunt (34), Shaun D. Jackman (33), David B Jaffe (22), Erich Jarvis (38), Huaiyang Jiang (32), et al. (55 additional authors not shown)
and also the previous Assemblathon paper. Also check out papers by Steven Salzberg and Mihai Pop on this subject, plus the references within all of the above.
There are many others which I can't think of off the top of my head, I'm sure others will suggest some