If you are running solids [ unless things have changed ] You would be adjusting them every several thousand miles or less. Performance is as good as it gets at first but slowly falls off over time. I have never run them but some friends did and it seems to me they were adjusting them fairly often. They require special cams in order to run.
ALLEY
Ironhead XL's came from the factory with solid lifters, but shy of those, there is no good reason I can think of that any modern street engine should have them.
Incredile noise in any street Evo B/T or Evo XL.
Scott