Depends on what it is and how often you use it.
We still rent just about every smaller piece we need such as an aerator, power-seeder, cut-off saws for paver work, and attachments for my mini skid.
In 7 years I still haven't had the number come to a point where we can justify buying some of that stuff listed above. For an example, aerations, we do maybe 2 days worth of them, so rental is a no-brainer.
That and there is no need to service that stuff when not in use as the rental house does all that.
The only piece I dropped the money on was a back-hoe attachment for the mini skid as no one in the area had one to rent.