Firstly, I must say I am a hardcore Neil Gaiman and Sandman fan, and I had been saving cash to buy this for a long while now. Getting it was a real big joy.
I have written a big, detailed review on this box's content on my blog, where such thing belongs. Here, I shall stick to whether or not this is the fittest way to buy the series.
I'll start with the fact that, if you haven't read the Sandman, I should read the Sandman. It is the finest piece of comics ever written, it's high quality literature, it's magical, it's big and world encompassing and it is also deeply personal.
So, if you want to get started on the series, I would recommend either buying this slipcase (if you are pretty confident you'll like it, which I am) or buying the first TPB, feeling whether it's your thing and then going on to buying this.
Two comments on those scenarios: firstly, there's pretty much no reason at all to buy the 10 TPBs separately. They are more expensive that way and you get less one cardboard box on your shelf. Secondly, if you are going for the buying the first TPB, I must warn you: the first Sandman volume is very different from the rest of the series. Master Gaiman was still finding his voice in comics and around his new brilliant characters, and the result is rather clumsy and unpleasant. Therefore, while the first arc is important for the story and the character development as a whole, if you are specially uncertain if you are going to like it and want to get some real taste of Sandman to decide, you should better buy Season of Mists (TPB #4). It is widely regarded as one of the greatest Sandman arcs, is relatively self contained and gives you a real taste of the real thing, so it would probably be your call (The Lord of Dreams going down to Hell to free his former lover and face Lucifer Morningstar's revenge for past offenses? Doesn't get sweeter than that.).
Anyways, for those of you familiar with the work and struggling to decide whether this is the way to buy the series, I'll present you with my two cents.
Firstly, this is the cheapest way to get all of the Sandman content (apart from Endless Nights, Death stories and extra stuff from the Absolute editions, this is just the core story), and a fine thing at that. This consists of the 10 remastered TPBs, and that remastering can definitely be seen. I'm one who read the previous editions, and can say for sure that the alleged recoloring was actually a very big thing.
Now, the option to this would be buying either the Absolute or the Omnibus editions. Considering you have the money and love the series, I suppose the Omnibus would be the way to go, as it presents all the same core content with a much better binding and amazing hardcover. The 4 Absolute ones didn't quite do it for me, as they are big, not as beautifully wrapped and feature kinda pointless extra content (that being said from a hardcore fan who would certainly LOVE to see meaningful extra content).
The TPB value shouldn't be ignored, though. Reading 10 thin, handy volumes is much more pleasant than reading two preposterous tomes, and that goes for lending your collection to people in order to spread word of this magnificent work (which is personally one of the main reasons I bought this).
Also, speaking from the condition of a Brazilian resident, this was the cheapest and easiest way to buy this, shipping cost wise.
So, that's what I have to say. If you wanna ask anything, feel free to do so at the comments.