Pros
National Grid are undoubtedly a good company to work for, in the sense that they will, by and large, look after you and although they've had to move with the times they still have a very benevolent streak. They are stable, so job security is high, and benefits are generous. There is no longer a final salary pension but what they do have stands up well in the market.
Cons
It's slow-moving, conservative and if you're ambitious, management is crowded and the inherent conservatism means that if you come in from the outside you will have to change your definition of career progression. It is not a commercial environment - a small core of 'commercial' people deal with the ins and outs of making money while everyone else is there primarily to keep things running and it is hard to connect with the business model. Its definitely not an exciting business. It is also very political. I've been in many businesses as an employee or consultant and NG is right up there. But I'd say that there's nothing inherently bad about NG - just think very carefully about whether you will fit in and enjoy working there before joining.