Fiddlesticks is hard, he never gains much survivability and becomes a completely different character when blink is up versus when it isn't. Also he forces you to really push the early to mid game, if you aren't dominating your lane to start its only going to go downhill. Haven't tried Singed yet, I wanted to but he's a tank and I've never played tanks in DoTa so I'm not sure how to do it properly.
I did play some Malphite yesterday and he's pretty fun.
Anyone got advice for Sivir? I like her skills but ye gods she is so fragile at least early on. Is she just weak early game or was I doing something horribly wrong?
Sivir is a pusher. You generally can do damage in team fights, but as you've already noted your survivability is non-existent. Her ulti is huge in groups though, and if you can get the other team to not murder you horribly (possibly by baiting your team, or having a decent tank) your glaives can do quite a bit of damage. Her shield works like a mini-Banshee's Veil, but i'm not sure as to the exact functionality of it - some things it blocks, some it doesn't.
So yea; she is excellent at pushing lanes and then tp'ing back to help the team, and can take towers very rapidly. She can do good damage, but is hampered by her terrible lack of range on the autoattack, and her low hp - which in turn unfortunately (at least in my experience) consigns you to just clearing creep asap and then praying that your team can do the rest of the work.