Lately I've been using triangle waves, and my thoughts are:
They have a nice timbre to them, but you have to be careful or something like a square wave at full blast in the same mix will bury it easily. Due to the fact that many sound chips featured tri waves with square and other oscillator choices that are louder than tri waves, tri waves aren't used to their full potential because of the hassle involved with volume levels.
Infamously, the NES/Famicom had tri waves available, but no volume setting. Thus they were underutilized on that system. On C64 its a challenge as well, but at least you can use an ADSR envelope for volume.
Now with modern trackers we can draw triangle waves and use them in more advanced ways, but the characteristics of the wave still require care in the the mix. If I have a triangle wave at v64, a square will drown it out unless its about 28 or under, but making it work can have interesting results.
Try a tri wave for a flute solo with quieter square waves providing a harmonic framework for the solo.
What do you think about triangle waves?