I just finished playing this, and I enjoyed it overall. And although it's a nice set of maps, map09 is my least favorite; it's too long (it took me an hour and a half), the fights felt repetitive and exhausting, with few ideas and set-pieces seen in the past two maps. Getting the keys felt like a real chore; they all involved doing the same thing for each key; enter door #1, flip a switch to open door #2. Enter door #2, flip a switch to open door #3, and so on... it wasn't that fun, to be honest.
In terms of bugs, I noticed few texture misalignment here and there. Plus, the Unmaker in the last room in map07 is assigned an ACS_Execute special but it wasn't referencing any script to execute.
One last, minor thing I'd like to comment on: these "one more to go" messages, involving switches, didn't feel necessary, in my opinion, because in most, if not all, cases, it was obvious what you needed to do. Take the first map, for instance. You have a red lock and a blue lock near the door that allows you access to the facility (or whatever it was). It's obvious that these two locks are necessary together to open the door...
Regardless of the above, again, I enjoyed this mapset, and I'm looking forward to Threshold of Pain 2.
The only mod I'm using is 
