At 25 quid I think the set is definately worth a blind buy.
As others have said series 1 to 3 are the shows best and Jonathan and Maddie had the best chemistry onscreen.
That isn't to say that series 4 was bad but it did lose some of the earlier magic that the show had, which I put down to two problems the first of which was replacing Maddie with Carla, not that Julia Sawalha did a bad job but it has to be said that Caroline Quentin was a hard act to follow. The second problem with the last series was the change and tone of the mysteries, instead of the quaint, suburban and rural settings of the first three series, series two instead focused its mysteries in more metropolitan, urban environments and this caused the series to lose some of its charm.
My favourites from the show are
...."Mother Redcap"