I have just finished a bottle of Springbank 21, like the tall one you have Scotchio. I bought a couple of bottles in 2001 at only £59.00. I still have one unopened which is definitely going to be drunk in the near future.
I don't know what you've paid for it, but if you bought it 5 years ago it should've been approx. £60.00, and for that price...drink it - no matter what you'll be paid if you sell it.
It's considered a classic, one of the ultimate Springbanks. It's got a fantastic sea-air nose, the colour is medium dark oak. The taste is salty with fruit and hints of coconut, a little toffeeish.
If you want to consider another Springbank I would recommend the 175th Anniversary bottling which can still be found at some online shops or the 10year old, 100 Proof. The latter is one of the better Springbanks bottled nowadays. It's sweet with a touch of honey, demerara sugar and vanilla.
The 15y. is also okay but not by any means near the 21y. you have in possession.
You'll regret it if you don't drink it...