I caught onto this series about the 3rd or 4th season, and eventually downloaded every episode from every season. Highly addictive, although the series went from Great to Good when the plotting became less tight, and when it became more politically correct (replacing Islamic extremists with Russians; throwing in a Muslim iman as a token good guy, etc.)
But overall a quality show, and I look forward to the new mini-season.
That is about the time I started watching it.