There are very few game franchises around which have the kind of recognition that GTA has. Need For Speed is (or at least was...) one such franchise. While it didn't quite reach GTA's monetary success, it was one of those series that gained traction outside of the core gaming community. NFS became something of a casual game — not in terms of mechanics or quality, but in popularity and audience.
While many fans and certain critics think that the Need For Speed franchise has lost its way with recent installments — and a botched film adaptation — however recent missteps haven't ruined the legacy of the series. Many of the legendary installments in the franchise are still loved by the fans.
Need For Speed Underground is something of the "GTA San Andreas" of the NFS series. It's an older game considered by many to be the best in the entire series. It deviated from the professional racing theme of the other games and moved the setting to the illegal street racing scene.