FWIW, at work I use Pro-Engineer, Microstation, and Autocad, but for Home and Garage design work, I use Home Designer Pro 7, from Chief Architect. It's pretty intuitive to learn and has great tutorials for the uninitiated.
Sketch-up is probably quicker for general brainstorming, but HDP will do full detail drawings from your 3d design. It took me about 1 hour to do the layout below. Mind you it's just rough layout, and all the details will take a few more evenings to fill in (while enjoying some Top Gear re-runs and frosty cold ones

)
<a href="http://www.flickr.com/photos/12009583@N06/2085706208/" title="shop5 by npulliam, on Flickr"><img src="http://farm3.static.flickr.com/2405/2085706208_238c0da309.jpg" width="500" height="340" alt="shop5" /></a>
<a href="http://www.flickr.com/photos/12009583@N06/2085705878/" title="shop3 by npulliam, on Flickr"><img src="http://farm3.static.flickr.com/2195/2085705878_0cc260933b.jpg" width="500" height="342" alt="shop3" /></a>
<a href="http://www.flickr.com/photos/12009583@N06/2084921527/" title="shop1 by npulliam, on Flickr"><img src="http://farm3.static.flickr.com/2155/2084921527_f36c5a6907.jpg" width="500" height="342" alt="shop1" /></a>