I have one, it's ok. It's been on and off the car a few times. I do notice a little better throttle response. They tend to attract alot of attention, I've seen people turn all the way around in the seat to get a look at it. It's not overly pretty and you'll have to drill holes in your bumper cover to mount it.
Now for the facts. It picks up alot of road debris, which collects in the airbox. If your filter is sealed properly, it can't get into the engine. I've found bugs, cigarette butts, leaves and such in the box, but not so much as a bit of sand beyond the filter. The filter needs to be cleaned more often. As for water in the engine, it's a long odd distance from the scoop to throttle body(water doesn't really like to flow uphill). I can see it possibly collecting 'road spray' from say a semi, but I haven't noticed the oil being 'washed' off the K&N, so maybe not. I do avoid big puddles, but that's just SOP for me anyway.
I paid $40.00 for mine used with no filter. I don't think that was a bad price. I wouldn't pay full price for one, but that's just me.
It's not a mod you can really dyno test either, but March claims 10-15hp. Seems about right since the system looks similar to the setup on a 68 W-30(?) 442(it had 2 scoops), and GM said that was good for 10hp gain.