I've had my standing mixer for a couple of years now.
As far as paddle attachments, I've got one metallic attachment, and a similar one with a rubber edge. I am making primarily blondies, cakes and cookies. I find I am using the attachment with the rubber edge pretty much exclusively (except for tasks like whipping cream, where I use the whisk).
When should I be using the all-metal attachment, and when should I be using the one with the rubber edge?
The all-metal attachment is the "stock" paddle. For a long time it was the only paddle KitchenAid made. The one with the rubber edge is designed to scrape the bowl more effectively, and is an addon for the higher trim level of mixer or sold separately.
My experience is that the all-metal attachment is decently good at minimizing the unscraped part of the bowl. Because it's entirely rigid, and because it and the bowl were designed for each other, it can get very close to the bowl without colliding with it (particularly once you've adjusted the grub screw for that). The gearing leaves a slightly washboarded, but overall thin, layer of unscraped food.
Whether the one with the scraper is better really depends on what you're cooking. Honestly, even when I'm using the scraper I sometimes find myself stopping and scraping down the bowl with a spatula once or twice, and if you're doing that it doesn't really matter how well the paddle scrapes. And for loose, non-sticky mixtures you probably don't even need to do that, with either paddle.
I would only reach for the scraper paddle for something like cookie dough where some of the butter could stick to the bowl and never come un-stuck. Basically, if the non-scraper paddle isn't leaving particular individual ingredients on the sides of the bowl, the scraper paddle isn't necessary. And if my scraper paddle broke, I doubt I'd buy a new one.
