It took me about 10 minutes of listening with a Sennheiser HD598 using the trial version until I purchased a license. The improvement was huge, and I spent the rest of the evening listening to favourite tracks, listening to entire tracks in most cases instead of just skipping about (which I think is always the best sign with any new audio gear, since you're not just hunting for short-term audio kicks but really re-discover musical pieces).
The strength of the effect obviously varies across headphones, but among the seven supported models that I own, all improve. The worst one (really bargain-bin Sennheiser HD201) become
somewhat listenable, the best one (Fostex TH-x00, with individual calibration) only leaves me wanting for a wider/more precise soundstage. It obviously can't cure every ill a headphone has, but it's damn good at the frequency response correction.
Seeing how much effort goes into getting a flat/neutral frequency response from headphones - and how much this raises the price - I think digital frequency response correction is the way forward. (I can't speak to this myself, by remember reading in one of Tyll Hersens Innerfidelity articles that distortion and flat frequency response are tradeoffs against each other in driver design.)
If you're into headphone listening, it's definitely worth taking a look at the list of supported headphones (
https://www.sonarworks.com/truefi/headphones) and download the trial version if something you own is on there!