I don't think they're great at all. Maybe great for devs, not for the end user. It makes it much harder to update a vulnerable version of OpenSSL for example in all your apps. Because every maintainer needs to do update the included version in their snap. In the apt system you just update the library and that's it, everything's fixed.

Also, the dynamic loader has much better performance if it doesn't need to juggle different versions. I can see the benefit for some packages that are super complex or have really special requirements. But not for every little thing like Ubuntu is doing.

But Canonical doesn't own apt. They own snap and the store can be run only by them. I think they just try to insert their own IP into mainstream linux so that they can eventually charge other distros and/or commercial customers for access. I think they're just looking for ways to monetise. Inserting your IP into the mainstream is a common way to do that when it comes to FOSS. It worked for RedHat though they are a lot more successful at it. I hate this kind of monetisation, though I wouldn't mind paying for a distro if it were made with my interests in mind. Ubuntu is going the complete opposite way though. They're serving their own commercial interests first.

It's not working out at all though because everyone hates snap and even ubuntu-based distros remove it. I hope they will give up soon, just the way they've given up on Unity, UpStart, Mir and Ubuntu Mobile.