I've got ACID Music Studio 7, and I'm starting to use it more and more. (Now that I finally bought it, I suppose I should remove the "evaluating" note in my sig.

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ACID Music Studio 7 is the less-featured version of ACID Pro 7, but you can do a LOT with it before having to shell out another couple hundred bucks upgrading to ACID Pro. One thing I really like is built-in MP3 export. At least one competing product requires you to purchase a plug-in for that.
Generally, I find the product easy to use, and it seems to work well with my keyboard. I haven't yet figured out how to use my keyboard's editor plug-in, but I suspect I'm missing something obvious.
Here's a
link to the ACID Music Studio product page. You can download and play free for 30-days.