I think I caught the tail end of Galactic Cowboys set when I saw DT for the first time, but I wasn't familiar with any of their music, so I couldn't make a judgement call either way. I missed almost all of I Mother Earth's set in 94, but what little I heard didn't do anything for me. Saw Big Wreck twice and neither time impressed me; it would be interesting to go back and listen to their music now to see if I'd appreciate it or not. I vaguely remember Full On the Mouth opening for DT in May of 1998 - I was more into them than Big Wreck, but still they didn't do much for me.
I managed to see Spock's Beard open for them several times in 2000, and they were the first band I really enjoyed despite not knowing much of their material. Actually, seeing them live moved me to pick up their (then) new album V. Also saw the Dixie Dregs open for them in LA both nights, and the music was enjoyable but not enough for me to want to check them out further.
In 2002, I saw DT with King's X and Satch twice and they were great to see, altho I already was very familiar with both bands. Same thing for the 2003 tour with Queensryche and Fates Warning - probably the best bill DT was on with other bands, altho I'd always taken Evening With shows over shows with an opener at any time.
I saw them open for Yes - does that count?
Caught 2 shows on Gigantour, altho none of the other bands besides Megadeth and Symphony X did anything for me. And even with Symphony X, I was disappointed the way they "phoned in" the backing vocals by using pre-recorded tapes instead of doing them live...just like DT's been doing of late *sigh*
The openers in 2007 were a mixed bag. Into Eternity was interesting to watch, and their lead vocalist was definitely talented altho I hated his cookie monster vocals. Those vocals were enough to turn me off to giving them a chance. Redemption's music was pretty good, but I remember them being pretty boring to watch live.
For ProgNation 2008, I made sure to catch 3's set, having seen them before opening for Porcupine Tree, and they continued to deliver. However, since I can't stand cookie monster vocals, I skipped both BtBaM and Opeth's sets.
I missed out seeing Scale the Summit for the one ProgNation 2009 show I saw, but Bigelf and ZPZ were both impressive. Bigelf was definitely more accessible to my ears, so I got more into their set than I did ZPZ. Don't get me wrong, ZPZ are extremely talented to play Frank's music; it's just that I'm not a FZ fan at all, so while the band was entertaining enough, the music itself did little for me, even tho I knew it took great skill to perform it well.
Didn't bother watching Trivium's set either time they opened for the same reason as several others - the harsh vocals were a complete turn off. But I did catch Andy McKee opening for them and he was very entertaining, both with his music and performance, and the inbetween song banter. The last opener I saw was Crimson Projeckt. Like ZPZ, I know the music was challenging to play and the guys were entertaining enough, but I'm not into KC's music, so their set fell flat for me.