Pros
Excellent training opportunities (both internal and external). Excellent colleagues to work for (for the most part). Good benefits (no bonuses, but the staff Oyster's handy). Great life work balance (also a con if you're a dynamic individual).
Cons
By public sector standards, TfL is not all that bad. Certain parts of the organisation are well led (normally by people from industry) and there is some real expertise around. However, the pace at TfL is very slow. Very few people (if any) do more than 3 days worth of work over a five day period. Prospects for progression are slow. Although salaries are not that bad, the opportunity to get involved in some really meaty pieces of work is limited. The strategic stuff is retained by the senior managers (band 4 and above), in a busier organisation senior managers would have to delegate more. Overall, not a bad place to gain some experience & skills but not somewhere you'd want to stay if you are ambitious, hard working, driven, relatively young with a desire to succeed.