Mike Carr who is probably the most highly regarded authority on water on Mars takes a dim view of their conclusions. In particular he disputes a key point that the valley systems formed at the same time as the early bombardment:

New view of Mars -- a battered planet
David Perlman, Chronicle Science Editor Friday, December 6, 2002
"IT JUST ISN'T SO
"But Michael H. Carr, an astrogeologist at the U.S. Geological Survey who has long analyzed those valleys, argues that Toon and Segura and their colleagues "have the timing all wrong."
"The huge craters, Carr said in an interview, are so heavily eroded now that their once-high rims could not possibly have been scoured away by water only a few meters deep and could not have been formed at the same time as the river valleys.
"It would have taken hundreds or thousands of meters of water -- floods and floods of water -- to erode the craters that deeply," he said. "And the valleys could never have been formed at the same time as the craters -- the craters are much older, millions of years older, than the valleys.
"The possibility of recent floods there, with water buried beneath very young lava flows, seems very plausible," Carr has said on the basis of evidence from the Mars Global Surveyor, which is still orbiting the planet. "
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2002/12/06/MN77334.DTL

This completely undercuts their argument that water could not have been there long enough for life to have developed.

Bob Clark