Okay, so I am sure that I never used Spelling Power to the upmost.
But, basically, it is a BIG book of spelling lists with the words broken up into phonetic families. It is multi-level and inexpensive when you consider that you can use it for several kids/grade levels. I think the most useful thing about it really is that it teaches the parent how to test the child and teach the list.
The way we did it: Pre-test, knock the ‘known’ words off the list, child writes the list out, studies the words by visualizing and saying the spelling, test at the end of the week, and carry missed words over to the next week.
It is a pretty simple system, but I wouldn’t say it is a repacement for copywork/dictation. When the kids do copywork, they are incorporating a lot more information than just the spelling of words (vocabularly, grammar, syntax, style, thought,etc…).
But, if you want a spelling LIST book than Spelling Power is a good way to go, IMHO.