Spell resistance does not work against Timestop, spell mantle doesnt either. Infact, nothing does. It -is- an I-win button. And as a Player who has frequently used it and had it used against me to both insta-win and get insta-destroyed in both PvP and PvE, I say..
Nerf it. Sithicus can be beat without it, we've done it before. Its a great safety net to have when things go wrong, it really is, but there are other ways to get through Sithicus, like bringing extra people who can take over if your frontline golem gets dispelled. All sithicus visiting people know what I mean there.
For PvP, it's pretty disgusting how strong it is, really. Completely, and entirely uncounterable (Except for counterspelling, which only works if you're also a ninth lvl mage with either TS yourself, or Mordenkainens). I'd be very happy to see it put invulnerability on everyone, because at that point, you can use it to buff and reposition, and its still a very strong spell, but once it ends and you can go offensive, people can actually react and -do- something to survive, which is alot better than just be dead with nothing you can do about it.
Edited for typos and stuff.
That's my 2 cents as the player of Natalie, who as Norture said, has indeed floored people with Timestop.