It's not too bad though Harald, when you're designing a theme or editing one, you get a pretty good idea when you're in the Designer how that theme will look in your shop. the only things you won't see are any specific items you have in your own shop as all the things in the Designer are just generic for you to see what it will look like. But everything else will show up on your shop pretty much exactly how you see it in the Designer. There's really not much guesswork needed, and I don't think I had anything come out differently on my shop than it was in the Designer.
After you get the main setup done and uploaded, you'll be able to tweak anything that might need tweaking such as moving margins if needed for your products, but even that can all be done right in the Designer without much needed after uploading.
The only things I had to adjust mine for was because I created external pages in HTML to match my shop, so I had to tweak here and there to get the Headers, menus etc. to line up closer. They aren't perfect right now, but that's ok lol. I got tired of messing with it and figure after Xmas I'll tweak it to be better
Since the Designer is Visual there's no need to see what it looks like in your actual shop to know what it will look like uploaded. Unlike many Visual editors that show sometimes different when uploaded, I can say without a doubt that the SCD has probably the best overall performance of WYSIWYG that actually shows you the same as you see in the program when uploaded. Quite a welcome and refreshing change from many of the visual designers I've tried in the past
Give it a whirl and see how nice it is and I'm sure it will suprise you too