I have chronic pain from a severe back problem that I've been dealing with for over a decade now. I have depression as a result of this which isn't uncommon. I took just about ever SSRI there is, except Zoloft and Paxil. But I did take Lexapro, Celexa, Cymbalta, Prozac and Effexor. I might be leaving one of them out. They gave me really odd very uncomfortable side effects, with Effexor being the worst and the last SSRI that I will ever try. I got the "brain zaps" from that one and I've never had such an odd and disturbing effect off of a drug.
So I moved over to trying Remeron (an SDNRI I think...), Amyltriptaline (a trycrillic anti-depressant I think...) and your drug, Wellbutrin. The Remeron and Amyltrip. didn't work but the Wellbutrin has been a great drug for me. I've been on it for about 6 years now.
So the usual thing, everyone's body is different, your experience may be different than mine, I use it for chronic pain induced depression not just "depression", if there's much of a difference, blah blah blah, but here's my experience with this, in my opinion, very good drug.
It doesn't work long term for me. What I mean is that it works for around 9 - 12 months and then it just sort of dies out and I become depressed and even a little more unmotivated than I was when I previously was not on the drug. This has been something that I have for sure found to be true over and over again on Wellbutrin. Because of this I use it in an on for awhile, then off for awhile then on for awhile, then repeat kind of cycle. I will take it like I said for about 9 - 12 months and then stop for around 3 - 5 and then slowly start back up from 150mg XL once a day, moving up over time.
When I start the drug I start on the 150XL once a day in the morning. It's main effects are a slight decrease in depression, a very nice increase in motivation and easiness to be content with things and know things "will be ok" and "all things will work out" and "this too shall pass", that sort of thing.
I slowly move up the dosing, after a month or 2 I go up to either 2 150mg XL pills or 1 300mg XL pill. I never take the 300mg SR anymore or take 150mg XL 3 times a day, I take it once in the morning depending on how much I take and I generally don't go over 450 - 600mg's a month. If I'm on 2 150mg XL tablets I might space them out by taking 1 when I wake up and the second one about 4 hours later, but usually I'm taking both right in the beginning. 600mg's doesn't last for long - if I'm at this much I know it's time to cut back a little and stop.
Generally if I'm up to the point of taking say 600mg of Wellbutrin and abruptly stop, something I try to avoid, I find that I get some nasty but very manageable side effects. It's not at all like the other anti-depressant SSRI's or trycrillic anti-depressants. It doesn't work on Saratonin and I think this is the biggest reason why it's easier to "withdrawal" off of. Saratonin seems to be a sensitive brain chemical to change around quickly. Wellbutrin is an SDNRI, a Select Dopamine - Neprinepherine, Reuptake Inhibitor. It works on dopamine and neprinephrine. So a good idea would maybe be to take a look into these 2 chemicals and see what their functions are in the brain because this is what your body will have to re regulate when you get off of the Wellbutrin.
I find when I stop wellbutrin suddenly that I am unmotivated the most, secondly I would say I have a little bit more of a worry mind that likes to think in circles and it's harder to calm down and relax and know things will be ok. I tend to get a little bit of emotional sensitivity (crying would be easier when I'm quickly stopping Wellbutrin for instance), but this isn't that bad at all, the unmotivation seems to be the worst effect for me when stopping.
If I take a couple months and slowly bring my dose down to zero, it seems to not do anything at all except make me depressed like I was when I wasn't taking it at all. It's a depression from chronic pain though and I really think this is a different kind of depression. I wake up from pain and I go to sleep from being exhausted from pain. It doesn't have to really do with relationship problems or the inability to be happy. I can be happy it just is a shitty situation I'm in so I tend to battle trying to feel happy on a regular basis.
So sorry if I'm getting off track with the answer here, I'm very familiar with this drug and I like it a lot so I thought I'd give the long story. If you were on 600mg and you are now going to 0mg, yes, i'm almost positive you will feel some negative withdrawal effects most notable lack of motivation, worrying/monkey kinda mind, depression (but not severe), and a general feeling of fatigue. Also I find that I lose libido when I'm not on the drug. It's harder for me to get turned on and it's harder for me to maintain an erection. When I start back up on the drug it does the opposite.
If you were on say 150mg XL and are going to 0mg then your not going to feel that bad of withdrawal effects at all imo. You might feel lack of motivation, a slight loss of libido, a little bit of depression, but i doubt you will even get the worrying I was talking about.
It's a great drug I think and it's helped me a lot, took me awhile to find out how to work it best and I've been using it for a long time now and consider it a must for me to manage my chronic pain. If you have more specific questions please ask I'll be happy to answer. Hope it all goes well. Peace.