I am a cabinet maker and also fit Ikea and Bunnings kitchens for my customers-
http://www.makeovers-wa.com
The biggest advantage with Bunnings cabinets is the quality of the particle board that is used. Bunnings use a HMR (high moisture resistant) furniture grade board, that's why it's core has a green tinge to it. The white melamine finish is also a better quality than that used by Ikea who use a much cheaper board. HMR board is usually used by cabinet makers when making purpose made kitchens.
Bunnings cabinets also have a solid back unlike the flimsy Ikea ones. This results in a much stronger cabinet that can be anchored to the wall in many positions.
Whilst the Ikea cabinets are not the best quality the hinges / drawer runners etc they supply are made by 'Blum', probably the best quality fittings currently avaliable! Bunnings fittings are perfectly adequate.
Ikea solid wood benchtops are great value for money if you want wooden tops. Note however that they do require ongoing maintainace with regular coats of oil. There is also a chance of the beech tops going 'orange' over time, this is a problem with Beech timber in general.
If I was fitting a kitchen for myself I would choose Bunnings.