I subscribed to the Slate subscription as soon as it was made available. Love them. In answer to your questions:
1. I only pay 19.99 mo. The 24.99 subscription price also includes the additional LX480 Reverb plugin. I have enough reverbs. When I started my subscription, I paid 19.99 via Paypal the first time, and each month, I pay 19.99. I can stop my subscription anytime, and start up again anytime I want.
2. This was my first experience using Ilok. Slate sent me the dongle free when I subscribed. When it arrived, I plugged it into a free USB slot, and went through the licensing process. After that, I forgot about it. No issues, problems or worries. I'd much prefer a system without Ilok, just on principle, but in practice, Ilok really isn't that bad, at least for me.
3. In Sonar Platinum, I've never had an issue with Slate plugins not loading. Each time there is a Slate update, I install it, and on the first Sonar startup, the plugins get scanned by Sonar. That's it. Contrast that to Waves shells, which I have to wait to be scanned on each Sonar startup.
4. I'm not sure why some people say that Slate plugins are memory hogs. I have a few songs in which I only used Slate plugins, on every channel. The resources are no higher than any of my previous songs, which do not use Slate plugins. There is one exception. Slates master buss processor, FGX, I think. That beast sounds great, but is definitely a resource hog.
In summary, I've been very happy with Slate plugins. I feel they are a great deal for the money.