The generic WB XL is known to be less effective for some people.
I've taken Wellbutrin for probably about 5 years, and last year (about a month and a half after switching to the generic) I started to feel kinda crazy. My depression came back full force, which was odd because I'd just eliminated a huge source of stress from my life, and I was very anxious and just 'crazy' feeling. My mom saw an article in our local paper about people having problems with the generic and she clipped it out for me.
Within a week of switching back to the brand I was feeling significantly better.
If you're having insurance coverage issues, see if your doctor will write for the 150 mg pills instead of the 300's (you'd take 2 a day if your dose was 300 mg). Since there's no generic for the 150's (yet, and hopefully not ever), there's a better chance they'll be covered.
for more info on brand vs. generic, see:
http://www.peoplespharmacy.com/archi...wellbutrin.php