The majority of seedlings and cuttings I start indoors are in homemade paper pots. The advantages of these pots are that they are free and you can plant the whole pot, paper and all.
Even so, I do find many occasions to reuse cell packs that once held bedding plants at garden centers. I've reused these cell packs for years. I have seen them go from sturdy and lasting 2-3 years, to extremely flimsy and so thin it's hard to get the original plants from the cells without tearing both the cell pack and the plant roots.
Not to be outdone, I still use the cell packs, even if I have to nest 2 or 3 together. Usually, my plants have such a mass of healthy roots, it's hard to get them from the cell pack without tearing many of the roots that stick to the side walls of the cell pack.
I have come up with a solution to this problem. I insert strips of material into the cells before filling them with soil. When it comes time to transfer the plants from the cell pack to the garden, the plants and all the roots are easily removed from the pack with just a gentle tug on both ends of the strip.
It takes a bit of time to cut these strips, but I save more time and aggravation by using them. There's no mess from having to knock the plant from the pack.
The material I used here is a sheet foam that would not be available to most of you. I had it on hand and it lasts about 5 years, so I used it. I have used various other materials. Rather than tell you what they are, I hope to hear suggestions from you as to what you think would make good strips. I'm always looking for something different to try.