SpinReWriter is good, and I would consider it among the best of the ones I've tried out, along with WordAI, in terms of it's word suggestions - but it's not without it's flaws. Unfortunately it messes up dashes and special characters, and it also has some incorrect words in it's thesaurus that you need to keep an eye out for (it will some times suggest plural words for singular words and vice versa, or the words will have incorrect suffixes on them, or suffixes that don't even make sense with a given word.). As a result, you generally have to do some cleanup on each spun article, which is time consuming and sort of defeats the purpose of using an automated spinner. It does work really well though for manually spinning - I like the interface, it's fast and convenient for that.
WordAI is slightly better for completely automated spinning imo, but it's pricey as hell, plus they nickle and dime you once you go over certain limits per month. Also, I didn't like doing manual spinning in WordAI, their functionality for that is lacking - I guess since they are assuming people will be mainly using their automated spinning instead.
You also need to be aware of the limitations of spinners - they don't understand things like movies/books/music titles, or brand names, or common expressions for example, and try to literally replace every word. You can actually make them output much better results though if you build up a good "negative keyword" list of words and phrases for them to not spin and ignore.