Pros
Get to work on challenging and interesting projects using high-end FEA and/or CFD software. If you want to learn and are willing to put in the time, you will definitely expand your knowledge (almost regardless of prior experience). Management strongly encourages a deep understanding of the ASME and API BPV codes used routinely on the job, rather than a "plug and play" mentality used by many companies in the industry. This is a perk that should not be overlooked if you truly want to know and understand what you do on a daily basis. Small company environment with open door policy gives you daily access to all levels within the company.
Cons
Pay is average for small companies (not great, not bad). The main con is the sheer number of hours the job demands during urgent projects, which can happen frequently in the oil and gas industry.