Hi kristian012, I have used a Keeley Java Boost with varying degrees of success.
For me, it worked best with a Strat, and in front of an Ibanez Tube Screamer, as an extra boost/goose/gain stage. When engaged, it provided a nice, throaty kick that was great for leads. Sort of a cranked Vox-y vibe (which was otherwise unavailable from my Deluxe Reverb.) But I don't own the Strat anymore, and have found it not to mate as well with my Tele or Gretsches.
In the last year or so I've been using a Barber Tone Press compressor for the same function, to goose/boost (goost?) the TS for solos. Brilliantly, the Barber has a "mix" knob that allows one to mix the amount of straight and compressed signals, so I really use the Barber more as a boost, and mix in a teeny bit of compression to fatten things up for solos. I'll also kick in the Barber with a clean sound (no TS) for the same. The distinction here is the JB added a different "voice" to the sound while boosting, the Barber is more neutral.
I still have the Keeley JB and will hold on to it since I may find it useful again in the future for a different setup, recording, etc.