Pros
There is a large variety of different products made by national instruments which can give you the opportunity to learn about a wide array of technologies. Since the average age of the employees at the company is pretty low you can have the opportunity to get very involved in projects early in your career if you have the motivation to go after these opportunities. There are a handful of very knowledgeable technical people there who are great sources to help get you going if you can find them. The working environment is casual and for the most part the company is pretty forgiving about honest mistakes.
Cons
The management at NI is the single greatest downside of the company. The company recruits heavily for a position known as AE (application engineering) which puts people in a program known as ELP (engineering leadership program). These people are basically expected to do the technical support for 1-3 years and then have to move on to another area of the company. A lot of times this ends up being Sales or Marketing but you also get a fair amount working in R&D. Most of the R&D management came through this path at some point and while they are not bad people they really don't have the technical or managerial experience to be in this kind of management role. This leads to a lot of frustration for the technical folks. Overall it seems like the company never figured out how to deal with the fact it had grown so much so fast and now is just floundering around trying to make the old formula fit the current situation.