Yes, worth saving up for or, like me, going to the outlets to buy one! I've wanted a Dolce and Gabbana signature LBD for a long time and back at Christmas when I was in Italy I made a trip to the outlet just outside Florence and was so lucky to find a style i loved and in my size. It's not a given that you will find one, I had been going for years and never found one at the right price or style, but perseverance payed off.
Today it's also the first day of the matchesfashion.com sale, so worth keeping a close look at selection of Dolce pieces going on sale in the n few weeks. I saw a similar one to mine on The Outnet (click o the link) this morning at 50% too - a really classic shape that will always be 'in'. And don't forget to go in store as well - perhaps wait to the end of the sales, as I was in there for a client last year around that time and they still had load of LBD's.
There's no point in saying why it's so eternally useful - to me it's the LBD. I love the figure hugging style, it's incredibly constructed inside and outside to fit your body like a glove (mine has a slip inside that literally suck you in). It's actually one of those dresses that look better on curvier shapes, as it really embraces a woman's curves, so don't be put off if you are not a size 6 and stick thin!