My TJ has had a Hyline Offroad vent panel for nearly 4 years and I can't imagine my Jeep without it.
There's a small pic of my TJ on the Hyline Offroad home-page here... (green TJ heading into ditch)
Hyline Offroad, LLC
The vents have made no difference to running temps but the heat coming out of the engine bay is unbelievable.
I have had no problem with water entering the vents.
With the engine running, the hot air blowing out the vents stops water getting in.
On a couple of occasions, the whole vent panel has stayed dry when driving in the rain because of all the hot air pouring out.
My vents were looking slightly faded after 2 years in a very hot/sunny climate and I had the powder-coating re-done.
Here's a pic of the holes I needed to cut for the vents.
After cutting the holes, I sprayed the whole area with a rattle can - yeah, I know it doesn't match, but the vent panel covers it up anyway
And from underneath... you can see more of my 'professional' rattle can job