I REALLY enjoy the assassination missions and they do earn you good money. Yesterday I did five of them and earned myself about two million credits. They vary greatly (anything from 20 minutes to an hour each) but they usually have an interesting narrative, which makes them more fun than some of the 'cash cow' bounty hunting found around mining installations, or the all out fighting, but smaller bounties, you get in warzones. Assassination missions characteristically offer a 150 thousand reward, but you also get the bounty for the target (usually a nasty anaconda) which ranges from 100 - 300 thousand in the missions I've taken (I've only taken about 15 so far). So for example, a successful mission yesterday was an elite npc anaconda I took down which got me 230 000 bounty and 170 000 reward from mission completion, plus all the lesser pirates you have to take out while you follow a trail of signal sources to find your target.
Having said this, I believe there are some bugs in the missions, which occasionally make them impossible to complete (see threads on missions) and this affects your reputation, which can leave you with no missions offered because you have unfairly failed the previous one. I have found this happens when I take on several missions at once, so my advice is to do them one at a time.
PS I'm using a vulture at the moment and although some people are good at kicking anaconda ass, be warned, these missions are no easy feat when you're starting out.