Looks stunning, has a good feature set overall. Samsung bloatware is unwelcome and not terribly useful. Battery discharges quite rapidly to be honest, they should have put a bigger standard battery in. The battery can be changed by removing/replacing the back glass though, so since you are bound to have that opportunity that may be a time to upgrade the battery as well.
There are some great accessories for this phone, so charge cases and add-on physical keyboards can give the device a further distinctive.
Unfortunately, as hinted, these Samsung phones are not built to last and are in many ways poor quality for the price they command. Incidentally, iPhone build quality is no better. If we were not treating these multi-hundred pound gadgets as throw away items would be completely unacceptable in any other industry. These metal bodied glass backed phones are a nightmare. The slightest touch or flex breaks them, Gorilla glass or not. The screens are also extremely flimsy. For devices that are carried around day in day out for perhaps a year or two, the fragility is quite ludicrous.
On top of that these Samsung's feature a very "retro" screen burn issue and the dreaded permanent "black spot" problem. These things again, should not be happening on such expensive products.
Blackberry PRIV has better specs in a lot of ways, looks well, but is also well made and can easily survive drops and impacts or even being dropped in the toilet - without a case. Nokia's also tend to be more robust, but are far cheaper than Samsung or Apple offerings. I bought the S6 second hand and was not impressed - so bought a Nokia 6 which is a far better phone.
Anyway, the S6 is def a nice looking phone, relatively good value second hand, has a good spec, but despite its price is not likely to be a long-term companion.