While people complain that the Elder Scroll games can be quite bland and you play an expressionless psychopath mass murdering your way to the top of society, Oblivion is a sterling example of one of my favourite things about PC gaming - mods. I have hundreds from total conversions to one that adds little street cats that wander around looking cute. Oblivion really captures the DIY aspect of PC gaming I love in a shiny giant studio package, allowing me to change almost anything i don't like about vanilla. Unlike console people, to merely review and make judgements based on vanilla is to miss the beauty of PC gaming. It's that kind of community that keeps an aging game fresh interesting and addictive.
As far as the extras in the GOTY go, whether you care about player houses shapes much of the DLC, Mehrune's Razor quest is a large dungeon, nothing mind blowing if you play with Lost Spires mod, but some very shiny loot. The Knights of the Nine was a fairly straight-forward quest, would call it a good DLC rather than an expansion pack as such. However although you have to seriously stab someone at the end, at least some of the quests have variety in what you have to do a bit.
However Shivering Isles makes this edition worth your money imo. It adds some much needed oddness to the game, the kind of thing where you pause to think 'wtf is that?' as some creature shuffles or leaps towards you. Without getting into the whole 'which elder scrolls is best', one thing i felt was Oblivion and Skyrim settled with standard pseudo-medieval europe fantasy world, whereas Morrowind felt quite alien, but with it's own culture to be read about and discovered. Shivering Isles brings that to Oblivion. The books are a delight, and it's nice to experience a daedric realm aside from the Deadlands (which while fun, never felt like a place people lived). All in all and bearing in mind what i said about the still quite vital mod community, I totally recommend you buy this game.