I've recently been exploring the use of the palette knife - something I've long thought I'm not comfortable using. In forcing myself to paint the majority of a piece with palette knife and only resorting to brushes very sparingly I've found the learning curve to not be that difficult, I'm quite enjoying the process!!
The speed of application and raw textural nature of working with these tools adds what I feel to be an immediacy to the work and I realised that I've instinctively been colour mixing on the palette with the knife anyway, then picking up those colours with a brush - hey why not take out the middle man and just do it all with the knife?!