I'm about 12 or so hours into the game, I've completed a ton of the side missions and half-way through the main quest line.
My opinion? I love it. I've always been a fan of the Elder Scrolls games (Morrowind being my favorite.) so I wasn't particularly bothered when I heard Bethesda was using the Oblivion engine for the sequel to one of my favorite RPG series. I was a bit confused by some of the lore they added (as well as sticking Apple 2 computers everywhere...) I quickly adapted and rather enjoyed some of the changes.
If we're looking at this from an Oblivion standpoint, I will say it works well, Bethesda chucked out the oh-so-annoying Persuasion Mini-game and vastly simplified the Lock Picking Game, making those situations a lot more bearable. However, there are flaws that I wish Bethesda has considered on addressing when it was making FO3. For starters, given that range combat is incredibly important, I wish you had the option of taking cover, hugging a wall, for example, or using sandbags or other debris for cover (You can duck behind stuff in game, but its very awkward.). Given the plethora of grenades and enemies wielding rocket launchers, its down right criminal that you don't have a feature to roll out of the way. Third, your character doesn't seem to strafe very fast, so you'll still be hit by incoming fire. Finally, aiming my weapons was a pain in the ass (Tiny, tiny reticule, not very helpful.) and was heavily reliant on VATS (though, perhaps thats was Bethesda's intention.)