It depends. Teacher writing during a lecture? Not really legible. A lot of the artistic kanji (calligraphy) is hard to read too. For example:
is
武
道
The shape looks the same, but I would have never guessed what they were without knowing what they were beforehand.
Usually most people's handwriting is legible enough to read, but those with bad handwriting can really give learners like us a really hard time...
Here are plenty of examples of handwriting
手書き - Google Image Search