That's good advice.
Think of it as being like comparing a CRF230 with one of those nasty 250cc Chinese things - maybe the Honda loses on the brochure, but in the real world there's no doubt which is the better bike - you're better off trading some bling for quality.
None of the brand names are inherently good or bad - but they'll all have stronger and weaker bikes in their range. For example, say that the Trek is the best value $800 bike, you'll probably find that the $1200 Trek isn't the best $1200 bike...
Learn about the group-sets and components, and it will make comparisons much more 'apples with apples' - otherwise you're likely to be overwhlemed by all of the info.
Pay cash and haggle hard. Don't be surprised if you find that they aren't keen to move much on the price of the bike, but use that as leverage to get extras thrown-in or at a super discounted rate (as a guide: even if you buy the cheapest adult's bike, you should be able to get a helmet thrown in for free if you pay cash).
Look for deals on supersceded models, particularly if you're spending $500+.
Hold off buying until after Xmas if you can.
Shop for your bike shop as much as for the bike (same as buying a new dirt bike, I guess).
Don't buy a bike from Kmart/Big W/where-ever - back when I was working in the bike shop, after each Xmas, the about 50% of the work-load was services on the decent bikes that we sold, with the other 50% being the cash-cow: Fixing bodgy rubbish that came from from department stores. It was often frightening some of the things that we saw.
At least a half dozen times per year, we'd have to send the not-so-pround new bike owner to return the bike because there were large sections of weld missing from a frame or similar! The two worst bits about that is that sometimes the owner had been riding around, oblivious, and that sometimes the manager at the dept store would argue with the customer that they were supposed to be like that!
Don't buy a bike in a box, unless you KNOW how to assemble it properly (and given that you've asked, you probably don't). Mechanically speaking, virtually everyone on this forum could easily cope with building up a pushie - but knowing the tricks to the adjustments, and having the specialist tools is a real black art.
And cheap bikes (even the good quality cheap ones) often have problems straight out of the box - most shops will be happy to help you sort them out, but if they assemble the bike, then they will sort them out before you even see the bike.
Further, a decent shop will spend the time to get the bike set-up right for you.
