There's no question that it would create a slight disincentive to work, but I do not think that it would have an enormous effect on people's willingness to work.
I think this effect would be far larger than many here seem to believe.
What used to be called "middle class" makes what - $60k a year? (median family income in the US). Now imagine a family of four getting $52k "free". Would they stay in their back-breaking dead-end jobs? You could say they might be more inclined to move into more satisfying perhaps less paying jobs to supplement their UBI but that might make it even worse by displacing less educated/qualified people currently holding those positions. UBI has a potential of creating a massive disruption in the job market and I'm not sure it would be a positive one. Remember we would still need a solid tax base to support the UBI. Wouldn't want to end up with e.g. 25% VAT/sales tax.
Another negative effect could be inflation. I know the theory is that it would be just replacing current welfare system, but for a lot of people it would be additional disposable income and depending on how they spend it we might end up with price changes that would create new social problems in housing, healthcare, etc.