I have had one for >1 year, and he is tied for my favorite fish in the tank. He lives in a 150 gallon tank with mostly peaceful tankmates, and I do see him all the time although some of the time it is just him hanging out under a ledge.
Stuff you may already know but worth repeating:
- They are not great swimmers and strong currents with no relief could cause problems for your fish
- They need at least one cave
- Mine was eating frozen food (mysis shrimp and small pieces of krill) when I got him, but I have heard of people having to use live food to entice them to eat. Mine will only eat a pellet if my aim is so awesome as to hit him in the head right above his mouth, otherwise he will only eat mysis and krill.
- They are slower type feeders, aggressive feeders are not a good match as tankmates for this fish. The betta may not get enough to eat.
- Small types of shrimp (sexy shrimp) will most likely get eaten, but larger types of shrimp like coral banded or a very large cleaner shrimp may be fine. I have had a very large coral banded shrimp in with my betta for around 1 year.
- Mine cleared out the bristle worms in my main display tank. I have not seen one in there in quite awhile and the tank was infested when I got it (still bristle worms everywhere the betta can't reach though).
- They are a fair hardy, disease resistant fish.
- Extremely small fish may get eaten (small neon goby, small clown goby).
I hope you enjoy your fish as much as I have enjoyed mine! I do think that one of the reasons I see mine so much is the choice of tankmates. When I had a one-spot spaztastic pigfaced foxface (many people enjoy these fish, I do not), I hardly ever saw my betta. However once I removed the foxface, the marine betta was out a lot more.
ETA: if you want cryptic, get a dusky brotulid!!! I think I have had one for >1 year, and I have seen that sucker 4 times total and that includes releasing the fish.