Good to hear...I've clocked close to 300,000 miles on two T5s running 33x10.50 tires. Overall they have been good boxes but the first one did have issues, but I've learned a few things and think they make a decent 5 speed behind an inline 6 cylinder. If you are an aggressive driver with a heavy foot I would look elsewhere though
In my experience they are very sensitive to what oil you run and ensuring it is topped up. I know some people run ATF in them and I ended up running Pennzoil Synchromesh fluid which the transmission seems to love, it shifts really smooth. If you go with a T5 be sure you check the fluid level and keep it topped up. My first T5 had one of those small drip leaks, that never looked major. I took it on a long trip up to Tahoe doing 75 most of the way and I actually overheated it pretty good climbing Donner pass which goes up about 7500 ft elevation. I initially smelled something a little off then noticed the jeep started to slow down a lot. When I pulled over I could tell the box was blowing fumes out the vent and felt really warm. After cooling down it was able to drive again but had a serious gear whine. But I think this was all largely due to me running it with low oil. My second box is leak free and going on 100,000 miles with no issues.
Second problem I had was the T5 has a bolt on the drivers side of the transmission (mine was a tamper proof Torx head) and it is the reverse/5th gear pivot bolt. My first gear box that bolt started to back out causing me to temporarily lose 5th/reverse until I figured out the problem. I ended up using loctite on it and it held firm with no more issues. The current box the bolt has never backed out.
Third issue I had was the input shaft bearing retainer, which is what the throwout bearing slides on, is a relatively soft metal on the Jeep T5 at least. Over time it can get scored and gauged which caused me clutch issues, but if you use a little disc brake lube or copper anti-seize (just a very little amount) it will help prevent that from happening. Or you can buy an iron bearing retainer to address the issue.
Yeah in retrospect that sounds like a lot of issues, but that's over almost 20 years and hundreds of thousands of miles of driving so I think it is a decent gearbox. And One thing that doesn't get mentioned much is it has a pretty good low first gear ration at 4.03:1
I've always wanted to add this internal brace to beef up the gearbox too, but will likely swap to something different if my current T5 ever goes down.
I had one seize up on my going 65mph and it scared the shit out of me. Could've caused a dangerous wreck. However, it was new to me at the time, and it's likely a slow leak caused it to be low on lube, which as you point out will lead to failure. Good to keep an eye on that!