I'm sorry, I have never been on Sertraline, but I have had some experience with Effexor XR, so I can at least share that with you which might give you some information. Please keep in mind that this was only my experience, and is different for everyone.
I was on 75mg a day for a year, and then spent 9 more years on 150mg per day. The side effects weren't particularly pleasant, but side effects never are, so I battled through the nausea, constant lethargy, fatigue, headaches and yawning (doesn't sound like a bad side effect, but it's not great to appear to be totally bored at everything your friends and work colleagues are saying!). After a week or so, the side effects went and a month later the meds started to kick in, and I felt ok. If my withdrawal experience is anything to go by, you have heard correctly; it was hell. Some people come off it with very few side effects and they are very lucky. It took me two years of coming off it, then going back on when I couldn't handle the symptoms, then trying to come off again. They were the worst two years of my life, and I still don't know how I got through it. Some of the things I went through while on the meds still remain with me today; irregular sleep, low libido, loss of memory, dislike of going out in public. And although I have had other bad experiences in my life, it is still the only one that makes me emotional every time I think about it.
But, as we are all different, it's a great med for some people. And I have to admit; I don't have depression anymore and it has been nearly three years since I stopped. And I honestly can't say that if I have that time again I wouldn't have taken it; when you suffer from depression, you will do anything you can to stop it.
If this is the med that works best for you considering the Imuran you are also on, then great. As I have never taken Sertraline, the only thing I can advise you to do is to learn as much as you can; understand what Effexor is, what it is going to do to your body and how it might affect you. At least then you can be prepared for what may happen. You only have to read the countless stories on websites such as these that while it can do a lot of good, it is a potentially dangerous drug; and you should know what you are putting into your system. This is a short version of my experience anyway, if you need any more info, please just ask.