The pros would be that you could save significant time each morning applying makeup. If eyeliner application is a major problem for you then it could be extremely helpful for you to have it always be there. If you have sparse eyebrows, or are missing them due to over plucking or a medical condition, having them tattooed on might be a great option to you.
The cons are quite abundant. The obvious one is that it is quite expensive. Depending on the area(s) done it can cost you anywhere from a few hundred to a few thousand dollars. A cosmetic tattooist can possibly make mistakes – and the result could be not what you were looking for, and of course you may wish to change the color sometime – it is sometimes possible to apply makeup over top but not always. There are a lot of reputable artists out there who will do a good, safe job on your skin, but you always have to watch out for those who are unqualified or do it in unsanitary conditions.
Something that is both a pro and a con is that while it is considered permanent – the color generally fades within about 3-5 years. Bad if you don’t want it to because it could be costly to redo, but good if you are tiring of the color and want it gone.
If this is something you are considering, please do your research. Ask for referrals and references from those you are considering and visit their workspace to make sure their equipment is clean and well-kept. You also need to think of what you are going to be doing in the next 3-5 years. Having thick purple winged eyeliner may look cool while you’re in college, but not so much if you are trying to land a job!