I figured I would know fairly early on whether or not it was worth it for me. It was pretty much an instant fix, so I see no reason why I would ever stop. Mentally I felt loads better, physically I look great. The only negative side effects I've ever gotten have been fixable (acne=time, skin care regimen; fatigue post hysto=dropping dosage by 20%). I think it's important to be honest with yourself about what you want, what you can reasonably expect, and the current state of your health. And it's important to share all of those things with the doctor providing your hormones so that you stay healthy and get where you want to get in your transition.
Conversely, I had a friend get their first two injections and then decide that T was not right for them. I don't like to question people's choices or decisions regarding their healthcare, so I haven't asked why. They did dive headfirst into transitioning and sought out an informed consent clinic in order to get quick access to hormones, so this could have possibly been avoided by discussing everything with a therapist or counselor in advance.
When in doubt, talk it out.