There's an activation switch - should be in the center dash panel.

Not really...  remember that this is an exhaust brake made by Jacobs - not
what's commonly referred to as a "Jake brake", which is an engine brake.
You can hear the difference when it's on vs. when it's off, but it's not
very loud at all.  I see no disadvantage to leaving it enabled all the
time - in fact, with it cycling more often, it may prevent the butterfly
from getting crudded up with soot and sticking when it's really needed.  And
since the activation of the brake is tightly controlled by the computer to
only activate when the torque converter is locked, it's operation around
town would be very limited, due to your speed and lack of TC lockup.

I have a 99 quad cab 4X4 with 6 speed standard. I would not pull my fifth
wheel  trailer without the brake. It is very satisfying to arrive at the
bottom of a hill and not have had to use the truck or trailer brakes, but
know they are ready when required...........still cool.
Not sure how the warrantee is looked upon if you install the engine brake in
an automatic. There is some noise heard in the cab, but that is minimal. The
switch is easily reached on the dash and can be turned on and the brake
reacts within a second. The brake comes with a manual that suggests using
the brake to speed up the warming of the truck at a cold start in the
winter. There is a warning not to use the brake on slippery roads as there
is a slight risk of locking up the rear wheels however, I have used it
coming down a hill after skiing and when you know how the brake works, I
think it is just as safe as operating the truck brakes. It would depend on
the conditions if I would use leave the Jacobs brake on during an "in town"
trip.
I had the brake installed shortly after buying the truck new. We do however
have a few hills here in BC. As I said, I would install a brake on my next
truck too.

A exhaust brake is a very good idea to install especially if you plan
on towing a heavy load around.  The exhaust brake will save alot of pad
on the brakes.  As for the noise, its not bad. There is an switch to
contol the brake.  All in all the exhaust brake is a good investment,
which will save you money on down the road.

Brian