We already discussed this in the past
here. However, the thread was closed for some reason or another, so I'll allow this thread to stay open and un-merged to bring a new perspective on the topic.
Yes, I do believe Ichigo, his nakama and the Gotei 13 have a chance at defeating Aizen. Already, the odds are stacked against Aizen. He's lost half of his Espada, and no one has died from the Gotei 13. On top of that, there are 8 Vizards whose alliances are yet to be mentioned, but it seems they like the Gotei 13 better than Aizen.
Aizen is well aware of the Vizards as well. Shinji directly confronted Grimmjow, and all battle data is reported to Aizen. So he is fully aware that the Vizard are out there.
The only trump card Aizen has is the fact that he possesses the Hougyuku. As long as he has it, he is infinitely dangerous. If they're able to get the Hougyuku away from him, then victory is imminent for the Gotei 13.