Perception of your writing
The majority of the people are used to the new spelling now, so I'd suggest writing in new spelling simply because it would appear strange to not use it. Most people should know you are no native and therefore they'll expect you to write in the new orthography.
As for those minority using the old spelling nevertheless, it's their freedom but I highly doubt that somebody would alienate you for that.
There are some edgecases like "Ketschup" (new) vs. Ketchup (old, both allowed) and "Delfin, Delphin" or "Fantasie, Phantasie".
With these words (you need to know) you can express your level of education, your knowledge about the etymology. There I suggest to use the old spelling intentionally.
When citing texts (e.g. Kant or Goethe) you have to use the original writing although it may be wrong now.
German population & new orthography
According to wiki the new orthography is disliked. Although this dislike should be against the people pushing the reform and not about those using it. In numbers of 2008:
55% against new orthography
31% don't care
09% pro new orthography
However, don't forget most people are not bound by any kind of rules about the language (only the countries employees must use new orthography), a lot of people neither change nor care for new spellings. Therefore I highly doubt those numbers, simply because everybody against a reform could be somebody against any rules for spelling.
For a non-native the most important thing to learn is the grammar which didn't change noticable and there the focus should be on inflexions. This is what makes you understandable :)