I bought my wife the Black & Decker VEC010BD (by Vector Automotive).
It's got an on-off switch and an advertised 300 amps.  I tried using
it last night.  I tried a bunch of different ways of connecting (none
were final connection to ground post with switch on).  I did get it to
spark when making a final connection near a grounding wire clamp
screw.  When I finished the connection with the on-off switch it
didn't spark.

It didn't start the car (Honda Civic LX) either, although I was able
to use it the previous day when the lights weren't turned off.  I
think the previous dead battery incident coupled with brief driving
(didn't really charge the battery) and another "forgot to turn the
lights off" incident probably meant little charge left.  I was able to
use one of my Schumacher Mity-Mite jump starters (no power switch
though) and it started it up the first time with the negative to the
grounding wire screw.

So as a recommendation, I'd say don't get the VEC010BD, which I got
for $35 at Wal-Mart.  I thought it was a deal (also came with a 12V
charging plug to charge the battery from the car and keep settings
when replacing the battery.  It's rather lightweight, which seems to
mean it doesn't have enough to start a car unless you're just barely
under the point where your battery won't crank the engine.  There are
larger B&D/Vector jump starters, as well as others including my
heavier Schumacher Mity-Mite (I think they have a newer version with a
different name - Instant Power?).

With the Schumacher unit, I've gotten desperate a few times (wouldn't
start when connected away from the battery) and made a final
connection at the battery's ground post, but I saw no spark.  Probably
stupid of me but I didn't know what else to do.  I did turn my face
away just in case.  I think in a real worst case scenario and I'm at
home, I could connect two (from different cars) in parallel with the
ground away from the battery.