I had a weekend job at Tesco (1pm-10pm every Saturday). Whilst the money helped with travelling, I wished I applied for a loan instead. I'm studying Architecture which is considered to be the most demanding and time consuming degree. Out of around 20 people I know at the Uni on my course, not one of them had a job or intend to get a job at all when they are studying. Plus, these students were getting lower grades than me and having more time on there hands! (lazy, night-time clubbers basically, who had parents to pay for it all).
If you don't HAVE to work, I probably wouldn't. If you can manage work and Uni, then definitely work, the money is worth it....