Lots of stuff about this topic here...
http://www.diyfaq.org.uk/plumbing/combi.html
Bath filling time appears to be poorer with combination boilers from what they say. With no stored water to draw upon; from a hot water tank, the flow seems to be limited by the boilers ability to change the temperature of the incoming water to the outflow temperature. Worse in the winter & better in the summer.
Peccavi
April 2009