I have been using my SD for 9 months, 525 km of mileage now. I love my bike very much!
The quilty of the ATS speed-drive is acceptable and almost maintenance free. Of course, it will be better if you can afford the Swiss-made Schlumpf speed-drive upgrade.
It took me some time to get used to the gear ratios and the shifting method. However, after you have got used to them, you will love your SD, especially when you are climbing up a long and quite steep incline. 
I agree with Mark Sanders that single speed suits Strida’s uncluttered, clean and minimalist design philosophy. It is already enough for daily commuting in urban area.
However, if you want to ride your Strida on different terrains, want a faster speed and do not mind the extra weight of the speed-drive and alloy belt pulleys, SD is a good choice.