Though it's called series 5, this is series 4 part 2 of Primeval. You can tell from the story arc, and there are some giveaways in the 'making of' feature- the scripts refer to these episodes as, for example, "series 4 episode 9" instead of "series 5 episode 2". It was clearly filmed as one thirteen-part series, and then split into two 'series' between filming and broadcast.
This makes a difference to the series as a whole- rather than having its' own original developing plot, it picks up where the previous seven episodes left off and if you haven't seen series 4, series 5 will make very little sense. So only buy this if you've already got or seen series 4.
The episodes themselves are great. Moving the show's production to Ireland gave it a new lease of life and it ends up being a very polished, confident drama. The special effects are mostly excellent and the acting is strong. It's a bit more predictable than something like "Doctor Who" but it's still good all-round family entertainment sci-fi.
The 'extras' (or rather, 'extra'- singular) is disappointing, as it's really the leftover bits from the 'making the new primeval' documentary that appeared on the series 4 disc, which feels like it's more aimed at trying to persuade new audiences to watch the show, rather than giving existing fans any real behind-the-scenes interest. Some more in-depth extras featuring longer interviews with the cast, or the writers, would've been very welcome.
As I write this the latest news is that "Primeval: New World" will be the next time we'll see Primeval on TV. It'll be American-made, feature a mostly or completely new cast, it'll "start fresh" plot-wise, and they haven't started filming it yet so it won't be on TV for quite a while. So while you're waiting, enjoy these last six episodes of the original Primeval.