I found them very effective. But I don't ask people to pay, I ask for feedback. If the software is good, they'll want to use it anyway!

People are willing to help if you are equally open about what the software can and cannot do. I implemented several features straight out of what people said they wanted through email, and the product was noticeably better for it.

I've also had people not respond at all or react strangely.

Someone on HN sent me a cold email recently asking me to help beta test their web app. They'd found me via a comment where I'd mentioned using a competitor's app, and wanted my opinion / feedback on their own app.

What they did right is that the first email wasn't a sales pitch, just asking for feedback / opinions / bug reports. What didn't go well is that they seemed rather defensive about my negative feedback. If you do get negative feedback, remember it might be specific to that person's situation - the same app might be perfect for someone else, and you can use the feedback to identify who is & isn't your target market.