It depends on your circumstances, the unions) on offer, your employer, arguably the position you have in the company, etc..
Worth realising that in a dispute with the company the one group that may take your side is a union, and avoids being forced to move on if unfairly treated. And that can be anything, industrial injury, unfair change of working arrangements, redundancy etc..
And there is a moral aspect since the union is (hopefully) looking after the interests of the workers (wait for the arguments to start below) so should you "piggy back" on their successes for free ?
Without a union your employer has a much freer hand at imposing what they wish, regardless of how it affects their staff resource. But you also need to look at how effective the union is or is likely to become. It's very much a personal choice, and there's no rush to sign up today.