Wines age much more rapidly in quarter bottles than in full ones - so keep an eye on vintages. Currently, best whites will probably be 2003/4, best reds 2002/3. Avoid 2004 reds for another few months.
Many quarter bottles are non-vintage. This doesn't necessarily mean they're bad - just more of a lucky dip.
Off smells and flavours? Complain politely after the first sip and ask for another bottle. Screwcapped wines aren't all in perfect condition.
Don't assume that because one wine from a particular producer tastes good, others will too. Quality varies dramatically, even within the same ranges.