I think a lot depends on your use of the flask, but there are some considerations.
Aesthetics: Brass is likely to corrode and/or get a nice patina, so that could be a nice effect. You're not going to get the same effect from a chrome-plated or stainless flask.
Health: Copper, a major compoenent of brass, can be toxic in quantity -- especially with acid contact. You certainly don't want to put anything acidic in the thing -- certainly keep your margaritas and daiquiris away from it! In any case, I wouldn't store beverages of any kind in brass for an extended period due to oxidation or other corrosion. Copper [lined] stills are still common, so it's unlikely that alcohol itself is particularly bad for copper. Further, nothing besides booze is meant for the inside of a flask anyway. :)
Maintenance: Brass will corrode and you'll need to keep it more well-maintained (specifically on the inside) than other materials.
Durability: In my opinion and experience, I have seen cheaper (e.g., chrome-plated) flasks corrode at joints/seams and get at least unsightly and perhaps unhealthy. Brass should be more durable than that, especially if properly joined/brazed.
For my money, I personally like high-quality formed stainless steel... though there are certainly pro/con tradeoffs for stainless also.