Shivering Isles is just what you'd expect an Oblivion expansion pack to be.
Yes, I bought the disc version. I'm still not sure what I think of download content on the Playstation store, plus I already own the Oblivion disc anyway so it only felt natural. Thus, I'll be reviewing the PS3 disc version, though I imagine in-game they are identical in every way.
Start-up process is easy enough. A few disc swaps on first play and then you can use the Shivering Isles disc to play the entirety of Oblivion and her expansion content. Loading times on and off (or rather in and out of) the Isles seem largely identical. As you might expect, this disc gives you more Oblivion and little else, if you could want such a thing.
What's nice about the Shivering Isles is that everything on the island is *new*. The place has a distinct look, and is populated with new creatures, items, clothing and people. Shivering Isles is arguably the best-looking patch of land in the realm, so it's worth seeing. The island seems to me roughly 1/3 of the size of the main content all around, and it takes a good bit of time to complete, even if you just rush right to the end... could vary wildly depending on your playstyle, but I'm guessing it'll take you at least a couple marathon sessions.
The story is sortof surreal (as you are on an entire other plane, or something akin to it), and climbing through the portal to get to the Shivering Isles immediately gives me the Alice-in-Wonderland vibe. It's the realm of the lord of Madness after all, so be prepared to meet some oddball characters, all of whom could stand in as any of the Tamriel town fools you've encountered so far. I rather like playing around with all the new stuff, gave me a reason to dust off my old characters. One nice thing is that you can carry the stuff you find in Shivering Isles back to the mainland with you, and vice versa. Although, don't expect too many Shivering items to start turning up in Tamriel... the core game is otherwise largely unchanged from what I've seen, which is disappointing. On my first playthough I found myself visiting the Shivering Isles in tiny bits, so I could digest the content at a variety of levels, and find/use all the new gear I found in the core game.
You can tackle the Shivering Isles at any level... as soon as you start it up you'll get a quest notice "Hey, a magic portal opened up somewhere. Better go check it out!" Kindof stupid really, but if they're just gonna hand you the expansion content, I'd have rather they handled it like the Morrowind expansions (where you had to ask around about them) or just broke down and said "thanks for buying the expansion, it's in Niben Bay, have fun!" For as much time as they apparently spent on the isles themselves, its disappointing that you are thrust into this story so artificially. Oh well.
The Verdict:
So, is Shivering Isles worth getting? Well, that depends. If you are an Oblivion addict, and you're just aching for something new to do, then Shivering Isles is exactly what you should get. Whether or not its worth $30 to you depends on how much you still want to play, but rest assured that Shivering Isles is by and large the same quality of content as the main game.
In fact, you can read my review as a testament that Shivering Isles in fact does not suck... so if you wanted to play but was on the fence about whether it'd be any good, worry no longer.
It's worth noting that the GOTY edition already has Shivering Isles, by the odd chance you didn't know that.
+More Oblivion
+New locale has distinct look, tons of new items and monsters
-Doesn't change the core game experience at all