[tzhenghao]

Short answer: Very.

Longer answer: I think one's skill level starts to taper off (or degrade / converge down to) the peers that they work closely with in time. This works both ways. Books and online sources can steer you in the right direction too, but generally with smaller effects. Approximately 1/3 of your day is spent in the office anyway.

Having worked at two startups now, both have very different senior engineers that have different philosophies on what good code is like. Neither camp is wrong, but I've learned to see things from another angle ever since. You then can bring cross pollination to the table to make contributions that one party isn't aware of before, or fill up a gap in your knowledge / skillset that you otherwise would've missed by not being exposed to others outside your common circle.

[atlasunshrugged]

Not a developer but I'll echo the sentiment above. I think especially in your career it is critically important to be surrounded by people you can learn from. This also implies (to me) that they are 'high performers' with something worth teaching and therefore likely to be promoted and/or do something interesting in the future and these are the type of people you want to be around for general career development and network as well.