Good, but Hard AI

Pros:

4/5 - Graphics: Most of the graphics go well together; menu graphics however could be improved.
3/5 - Sound FX: Pretty good for what the game is. I don't think this needs much improvement.
2/5 - Gameplay: It's tic-tac-toe! What else needs to be said? Good, but could use more features.

Cons:

- AI: I seriously suggest making an AI difficulty, because after playing 10 rounds the CPU had 5 wins and we had 5 draws.
- Popups: The windows dialogs that popup on lose/draw (I've never won) can be annoying. If you had it draw it somewhere on screen the overall feel of the game would improve drastically. I also suggest that for wins/losses you should have the winning x's or o's flash a bit just to emphasize the winning row.
- No Music: Although it's a very simple game, just some nice (relaxing) music can help calm the nerves and make the player want to continue.
- Interface: Since the entire game is played with the mouse, I suggest having buttons for "Play", "Options", and "Exit".
- Other: If you improved the interface, I also suggest you make escape from the game exit to the menu first.

Overall Opinion:

3/5 - Definitely could use some work, but the 3D looking graphics and animations make the game look good, feel good, and it's still classic tic-tac-toe. Maybe you could even improve on the idea by adding some extra features (maybe particles, another way to play tic-tac-toe, etc)