You can use it for free, without limit as long as you keep ads and captcha.
Is the text going through SpinBot.com ever saved?
No. Nothing going in or out of SpinBot is saved.
How many words per spin?
The per-spin limit is actually not based on words, but on total spaces and letters (keyboard characters). This limit is 10,000 characters per spin, which is about 1000 words (or an average length essay).
Is browser based time usage limited, or throttled?
Yes, but only for extreme cases. This limitation is in place to avoid abuse.