Depends what you shoot, how you shoot, the weight of your setup and the conditions you are shooting in. Opinions will also vary a lot on the quality and cost of the tools people can purchase as well.
Generally speaking a tripod is a help in almost all kinds of photography in a purely technical nature (that of avoiding most if not all camera shake). However they can be bulky to setup, slow to use and add weight to your setup. Tripods also have massive variety in the heads that can be used on them, which adds a whole new layer, not just the legs but also the right head for the specific situation. A geared head is ideal for macro work, but useless for action; whilst ball heads are almost the opposite.
Monopods provide an ideal, light support which can carry the weight without being too bulky.
In the end you'll have to work out what and how you shoot to work out what tools are right for your conditions. Budget is also important. Furthermore tripods are a bit like camera bags; you'll start off with one, but in the end you'll have a whole range of them and might well move through different models and brands as you work out what suits you best.