i like it much more than the above commentator, who inadvertently made some good points.
(a) cassiopia isn't an "oh **** it's... (op champs)" as such, a support role is good for her
(b) people don't focus her. for that reason, your description of the use of her ult is practially godlike.

in addition, i liked the build overall, and definently your mr guide. just please make a note on it that armor is pretty much the same.

I usually get focused very quickly in my match ups. Some people hope to initiate and CC me before I can get my ult off. When timed right that ult wins team fights. People try to stop that. My main goal with her is to make sure I get that ult off at the right time, and still be able to deal damage. Void staff would compensate for the loss of any magic pen as well, at which point you are only losing 9 magic penetration. Not huge for 35% CC reduction and some extra magic resist.

As for Clairvoyance I listed other spells. I just like Clairvoyance and as I said this build for me is what I use in random teams. You always going to have a support, intelligent team mates, team mates who buy wards, team mates who don't face check bushes, team mates who push top way to deep and way to long? You as a good player with clairvoyance help out your team more because you give them your insight. If I'm playing with people I trust to use CV yeah I go exhaust, teleport, or ignite.

Did you try the guide?

Sorry but, Cassiopeia has way better choices for summoner spells than Clairvoyance. Let some support champion take that skill. Also, why would you get 2 pairs of boots? If you mean we should sell sorc's shoes and buy merc treads at the end of the game, thats still ineffective. It will only lower your damage output. The extra M.R. on merc treads are not useful for cass as sorc shoes contribute more. Also, Cassiopeia doesnt usually get focus as in like... if someone sees a cassiopeia (with a score of like...... 0-0-0), theyre not going to go like OMG CASSIOPEIA, THE BEST AND MOST OP CHAMP IN THE WHOLE GAME, WE GOTTA KIL HER FIRST ! no... people dont do that because everyone knows cassiopeia isnt as strong as an Annie or Leblanc. so she isnt always focused.