It depends on many, many things, so I'll just give a basic overview.
First of all, there is a pure simian line, or a partial one. If you have one line, straight across, with no evidence of any separate head and heart line, it's pure. If part of the line branches out to the head or heart area, that part is magnified.
Alright, so basically, a simian line is when the head and heart lines merge. It can work well to your advantage, or well against it. People with simian lines can really get into what they are doing, they involve their heart in matters of the head, so can really put passion into whatever they do. However, it can also mean that if they are not really into something with their heart, it can be harder to wrap their head in it, eg, get bored of school or work and such easy if they aren't passionate about it. These people tend to be really smart because they can see the world in a different way than other people, however they are also more likely to go the way of mental disorders, like depression, bipolar disorder and whatnot.
People with simian lines know how to get what they want, and when they know what that is will stop at nothing to get it, and have such a strong driving source. However, if the want isn't there, the drive is nil. They can also get quite moody, especially when their needs aren't met.
Basically, the simian line is often a gift. People who have them really have this extra helpful connection between their heart and mind that makes them able to accomplish a lot. Whether used to create new helpful inventions, or become a serial killer, a lot of power comes with it.
Although this is just an overview, to really know how yours personally effects you, a lot more needs to be considered. Once again, whether or not it's a pure line, as well as if it is on both hands, what type of hand you have, what other lines will change it's effect, it really is much, much more complicated. But basically, it doesn't make you a failure, it isn't necessarily bad, but like everything it can be.
Palm reading for about 6 years, and I have a simian line (not pure though) on my left hand :)