I have just a small (compact one). Ideal for me as it does two good sized portions - so,either me and someone else for meal, or just for me for two days.
I used to go along the lines of following the recipe to the letter, and using a large frying pan (or wok) to sear the meat, and do onions, etc prior to putting into this. Then I watched my daughter who just placed everything in her slow cooker straight from fridge, etc and all cooked perfectly. So that is what I do now and cannot discerne any difference in flavour at end.
The one my daughter had I originally purchased in a Sainsbury's sale about ten years ago, and found it not to my liking, so gave it to her and she does wonderful meals in it. I was inspired to purchase for myself the small compact one, and it is great in the winter months.