I tried it for a while. Did it back in 2004-2005, and it definitely increased my libido, and it certainly made me "feel" healthier for a while.
I didn't see any real weight loss from it, though. Thankfully, no hair loss from it, either.
I'm currently 43 (so, I was around 33 when I was doing TRT).
I looked into doing it again a few years back, around 2011 or so. The problem was that all the blood tests they had me do, both before and after the testosterone therapy, indicated that my testosterone levels were ridiculously low -- like, to the point where they didn't really understand how it was even possible. Which doesn't make much sense, as I'm perfectly healthy and in great shape and have a very good libido to boot. But they felt that with those results they couldn't keep me on TRT therapy.
So, I pretty much gave up on it and wrote it off as their blood tests being invalid. But frankly, I haven't felt the need to get back on it again, since I'm perfectly healthy these days.
So, my take on it is that if you really have the symptoms of low testosterone, maybe it's useful. But it's probably not very helpful compared to what you can get from diet / exercise / supplements / healthy lifestyle / etc, and certainly won't extend lifespan.