In reply to Climbing Pieman:
I used to sell boots for a living and have used leather boots for decades (all non gtx). Nikwax is crap, is soaks in quickly but washes out quickly too. I had numerous people trying to return non gtx leather boots (mostly meindl borneos) that they claim were leaking despite regular application of nikwax, Meindl wouldn't take them back unless the workmanship was faulty (which it very very rarely was because they make top quality boots) and they only recommend the use of g-wax of meindl sportwax.
I occasionally use nikwax to soften up a pair of boots that have been neglected, but before using them I give them a good going over with G-wax.
Also, Nikwax now seem to only sell the rubbish cream (comes in a tube rather than a tin and is very thin).
Regarding the use of normal shoe polish, I wouldn't. We had some boots returned that were cracking and the maker (a well known italian boot company) said they wouldn't accept responsibility because the boots had been polished with regular polish that didn't preserve the leather and actually made things worse by sealing the leather from the outside so that it dried out and became brittle.
Make of that what you will.