Pros
Work-life balance prioritization is probably unmatched in the industry, especially for US employees. This extends beyond benefits but to (mostly) the management culture (you are required and encouraged to take all vacation, etc). As a young analyst, I had opportunities to see clients immediately, work on projects I was keen on, move across commodities, had unimpeded access to top analysts and managers, and was almost always rewarded consistent with my effort. Even after much reorganisation, WM is still a quite flat organisation and highly meritocratic. There is not an up-or-out culture and you really can develop as quickly (or slowly!) as you want.
Cons
In 5 years I was on a fully-staffed team 6 months at the longest - this attachment to lean teams is nearly indifferent to oil prices and corporate profitability and simply reflects poor HR functionality and management short-termism. The technology support is even worse - why analysts paid six figures have to manually enter data into multiple databases because no staff has time, expertise or authority to install working macros is still a mystery to me. Lastly, the quality of first-tier management is poor; this is a problem of desperation hiring but also poor training, broken feedback loops and too-long grace periods for poor-performing managers.