It is nice if you are lucky and the installers weren't cheap. I've seen a few of the places that use Cat 5 to service phone cabling do crazy things in the walls like split twisted pairs to different wall plates and bad cheap phone transformers (that grossly bleedover noise across the twisted pairs, entirely removing the benefits of twisted pairs in the first place in some cases) in long Cat 5 runs.

There's a lot of things that made Cat 5 cheap for running phone lines in houses and most of them make Cat 5 useless for Ethernet, unfortunately.

You can actually use old-fashioned four conductor phone wiring for Ethernet, but it's limited to 10 mbps. Still useful in a few situations if it's all you have available.

My house has all cat5e, and I've been very happy with it. Much faster and more reliable than wifi.