I watched the first season but haven't got round to watching the second. It's pretty enjoyable stuff even if it is pretty ridiculous at times but that is all part of its appeal.
Bit of a meh première for season three. The battle at the end was ridiculous. Don't like it when they have to make the antagonists into simpletons just to make the protagonists look good.
Any Vikings lucky enough to survive Season 5 shouldn't rest too easily because it was recently announced that more massive battles are being set up for the sixth and final season.
There's also a spin-off in the works to continue the story of the warring Vikings beyond that point.