Lots of people really like the Coleman, and it does have some advantages, but I recycled mine because I couldn't donate it with a clear conscience.
First, although compact, they are somewhat heavy. Second, there are several ways to injure yourself using one: failure to assemble it properly resulting in collapse, hot surfaces, and sharp edges (got me, and I am careful). Third, maintaining a stable temperature--especially in windy conditions, requires constant monitoring. I used an oven thermometer to help with that. Fourth, I question the safety of using one on a butane vs. propane stove, and I like having that option sometimes. Yes. I was able to actually bake in it, but what a hassle!
The Omnia? Not a fan, either. Too much hype, and too little cooking surface. Too pricey for what it is, IMO.
I have a stove top Westbend Ovenette and can bake anything that will fit in an 8" or smaller pan. It's super nice to pick up a small frozen lasagna or pot pie for dinner, or bake something "fancy" like a casserole or cinnamon rolls (LOL). Once the temperature stabilizes, it's no fuss. Although it doesn't fold flat like the Coleman, at 8-1/2" tall, you can store your cookware inside.
Some members of this forum helped me make the decision to purchase it, and I couldn't be happier with the choice. (The only issue I have is that steel components (inside) are beginning to oxidize somewhat.)
The downside? Hard to find. They were very popular wedding gifts in the late '40's until about 1960, when an electric version came out. Mine retailed for about $8 then!
Next best thing? I've gotten by with a set of pans, with one inverted for the top. Not great, but it can get the job done.