That's your call. I open all of mine. Sometimes you get a really expensive item and sometimes you get one only worth a few thousand. It's the same as any of the giftboxes or other rigs - you have to decide whether you want to sell them for the price they're going for or take the risk of getting something cheaper for the possibility of getting something more expensive.
Opened it!, and got fish eggs. Apparently it's ~rare~. Lucky me, it's just about the worth of the cache itself. XD
I listed it in the MP for 250k I think but I'm gonna go there and relist it for lower now, since I just realised that the same way I was desperate for some cheaper ~expensive~ components when I wanted to craft something, someone else might need this component too.
I have one that I painstakingly alchemized. Do I crack it open or do I sell it?
I've gotten two Philosopher's Cache's since I started gaia. The first one I got I sold for around 150,000 gold. The second one I got I opened it and I recieved 1 Mirror Shard. I've seen those sell for around 1,000,000 gold each. So, depending if you're going to sell whatever comes out of your Cache you might want to open it.