I was on a team with one other guy working on a complex distributed-systems project. We were both principle engineers at the company. We delivered the project in under a year and as far as I'm aware from talking with former colleagues, the code is still in production without issue years later. We paired every day for about 11 months.
Yes, its mentally exhausting. You need to take regular breaks. You also need buy in from everyone involved, if one person is half-assing it, its going to fall apart.
The benefits I saw were many: better quality code (two sets of eyes on the code at all times, two people solving every problem or bug, always someone to ping ideas off), better understanding of the code/knowledge sharing (both of us fully understood every line of code written, why it was like that, what it was for), shipping on time (easier to make decisions to get it production ready quickly when two brains are working on it, easier to avoid bugs, boring tasks like documenting are easier too).
I don't think the project could have been completed in the time it was with only two people if we hadn't paired. There was also no disparity in experience or seniority, we were both quite equally matched.