Pros
If you are a Principal beamline scientist or Group leader, you are allowed considerable freedom in running your little fiefdom. Some might say, too much. The working hours are decently flexible, though a bit dependent on your line manager. The offices around the ring are a great working environment. The site is likely to become the UK's main big science campus. May be a good choice for more junior staff if you're a non-EU citizen who wants residency in the UK. If you can get in the right group, a DLS postdoc position would be a good way of trying out the Big Facility lifestyle, which is different to a university. The new CEO seems to have fresh perspective.
Kontras
I can only concur with some of the other reviewers. DLS is NOT a career destination for an ambitious young scientist. Early career researchers applying for beamline scientist/support scientist jobs should be aware that there is a sort of demographic bulge of staff above you in the 35-45 age group, who have started families and have no inclination to move on - this leads to a lot of stagnation in the scientist roles. It does happen, people do move up, but there is a strong tendency to parachute in external big hitters. In particular, the Support Scientist role should be avoided. It is not at all obvious from the job specifications on the website that this role is a dead end and potentially career ending for a young scientist. The Prinicipal beamline scientists are given considerable freedom in running their ship, and there is no standardisation or oversight from senior management. Some PBS' are excellent and natural born leaders, others should not be in a management position. Any engineers or programmers wishing to join DLS need to be aware that the PBS' are allowed to act as project managers. There are a number who may be academically gifted but may be a bit clueless in managing large projects. You will be expected to implement their orders, no matter how insane - you have been warned!! There is a union, although it appears to be fairly weak by public sector standards and consists mainly of people from technical rather than scientific background. In short, if you are a young scientist looking for that first permanent job, carry on looking. There's better opportunities that this. Give it a try as a postdoc though!