Connecticut is one of the bluest of the blue states. There was widespread opposition to the new law to no avail. Though there are many gun owners, there are probably not enough to make a significant difference.
The law goes into effect in a couple of days. My sense is that there will be about 25% compliance on the long guns and much less on the magazines. In many ways it is unenforceable since magazine purchases have never been registered and direct private transfers of at least long guns did not require any paper work until now.
Early on there may be some arrests of people who publicly and loudly stated that they would defy the law. But in the long run it will be unenforced unless the violation is uncovered for some other reason.
In the future I think we can expect one of or both of the following, an occasional amnesty, as they do for taxes, and eventual confiscation of anything that has been registered.