There seems to be a strong negative emotional reaction among (some?) native German speakers to the new spelling rules. They use the old spelling rules themselves and dislike the new spelling.
Should a German student ever intentionally use the old spelling rules? Would this endear them to native Germans, or would it just look like the student had studied with an outdated textbook?
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.
Intentional writing
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.
Citing
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 :)
Don't try to use the old spelling if you are a foreign student.
Partial use of the old orthography will endear you to no one. Neither is it likely that the occasional "daß" will make you look out of date. Many people have continued to use "daß" while not applying most of the other concerned old orthography rules because most people never knew them.
But the simple truth is that for more than 15 years students have learned the new orthography, most texts are printed in new orthography, and it will be practically impossible for you to learn the old orthography properly.
The reform was heatedly debated more than 15 years ago, not now, and I suggest not to listen to people who emphasize their high level of language sensitivity by comparisons of rape and orthography reform, but YMMV.
In addition to what the others answered, I wanted to add, that if you're a student in a German course that started after, I think it was 2005ish, it would be plain wrong and graded as such, if you'd use the old spelling rules. As it is in German schools.
This also applies for any official exam, such as TestDaf, the ones from the Goethe Institut, etc.
I recommend you to learn the old, unreformed orthography. You are right that the majority of Germans was strongly against this violation of German. Not sure how is it now ...
If you want to learn Kurrentschrift, you also have to use the old orthography from Konrad Duden. It does not work with the refomed German orthography, because of the Lang-s, Rund-s and Scharf-s. It is impossible to write "dass" with double-s in Fraktur or Kurrentschrift.
