I could never get into using Sketchup for Forge maps. In the time I spent making the map in sketchup, I could have already made it in Forge. I also learn what won't work much faster, and allows a map to form more organically for me.
That's just me though. In my personal opinion, there's a lot of Forgers that make some nice ideas in sketchup, soak the praise for the idea, but then never actually make a map out of it. I prefer seeing maps in the engine first. A notable lack of Sketchup for me is it's a poor tool to determine your sight and firing lines from.
We all have our preference. I actually use the forge pieces as a reference. How I am doing it is start with shape/layout creation on sketchup, make a rough draft in forge, then use the forge pieces on sketchup to make something much more accurate while using forge as a scale reference, and then finally transfer it to forge and adjust for texturing and minor piece details.
I find it to be a much more efficient process because there are times where I need to shift a whole section of a map by 0.2 or 0.5, or 4 whole units and I have to inch each piece individually in forge. I can do it all at once in sketchup and much more accurately. Then if I find I don't like it I can undo back to where I started. Sketchup has been wonders for quick iterations. If I have a couple ideas for a section I can dupe the whole map at once, then make to different versions and decide which one is best from there. I just happen to be very nitpicky when it comes to precision.
As for LoSs... that is what Unity is for. A quick export and a FPS asset package and I'm running around checking sightlines. For presentation purposes I have to go through and remove z-fighting. But once I do I can host it and show off just the layout. This allows me to share the layout with people not on the xbox rather easily, and because of the generic texturing it allows people to focus on the layout over the aesthetics. It also helps drive in the fact that this is a work in progress, not a final product. I always love blocked out maps.
This is the older version of Think Again:
http://think-again.comuf.com/. It's has changed a LOT since this version and I should be ready to put out the new version in a couple days.
I'm very happy with the results. I'll probably quickly add in a quick feature that will allow people to switch between the two versions to see the changes.
Also having my map accurately built out on Sketchup allows me to have a direct reference when building it in H4 rather than building from memory from a map I designed on Reach.
Bonus side benefits.