People are making this comparison presumably because Hitler justified some of his earlier aggressions on the grounds that the annexed territories were inhabited by ethnic Germans. (He would, of course, have preferred the term 'race'). Up to this point I would agree that there are some parallels with the current situation.
However, the problem is that if you make that comparison you are also implying a lot of other things that don't make as much sense. For example, Hitler's early annexations were a mere prelude to a wave of aggressions that eventually developed into a world war. Now, whatever you think about Putin it is hard to argue that he will eventually invade France once he is done with Crimea or Eastern Ukraine.
Another rather big problem with the Putin-Hitler comparison is that it ignores the events that led up to the annexation of Crimea. Putin's action was a reaction to a geopolitical expansion of the west. After the change of government, Ukraine was bound to become a more pro-western nation with closer ties to the EU and perhaps eventually to Nato. This had happened before in other Eastern European nations and seen in this context Russia has been loosing a lot of ground in Eastern Europe over the last decades. There was no similar geopolitical pressure on Germany before Hitler started his expansion. One might argue that the Versaille treaty had led to territorial and other losses. But there was a substantial delay between Hitler's aggressions and, if anything, diplomatic pressure on Germany from the victors of WWI had been reduced in the period before Hitler's aggressions.
On the whole I think this is a really weak historical comparison that does not only reflect a tendency to compare anything you don't like with Hitler, the Nazi and/or the Holocaust but also a really distorted analysis of what is going on in Ukraine.