Each store is different.
Supercheap - good for cheap quality tools, specials on oils and things, pretty much anything of crap quality and low price. It can come in handy when you you're doing something and need something cheap and quick to finish the job. The oil's and things are only cheap on special.
Autobarn - Like WW said above, good for audio stuff and the bigger stores have a huge range of products. Autobarn's also I think the best for car accessories, ie seat covers, gear knobs, etc. The Autobarn's with spare parts counters are just as good as Repco, Autobarn in Narre and Dandy are awesome stores.
Repco - Generally trade quality tools and products. Every tool I've bought from Repco has served me beautifully and their spare parts counter is a treat. Also it's pretty bang on with the Autobarns with them.
Bursons - The best prices, the biggest range (except they don't stock Penrite) and bloody fast delivery. The catch is that you need to know someone to get trade prices. Even the quality of their own products is pretty damn awesome. Plus I've just found out that Bursons give preferential pricing to different customers, so it's possible to get things a **** load cheaper than trade price, as I have quite happily found out.
The quality of the staff can be ranked in the order above. Supercheap generally employ 16 year olds whereas Burons/Repco have much more knowledgable staff. Bursons are a mile ahead of even Repco though.
Autobarn is somewhere in between, depending on the store.
Secret to buying parts cheap. Get all your oil's when the parts places have 20% off storewide. It usually works out cheaper than trade.