Absolutely perfect is the short answer. Concrete is often likened to liquid stone and for good reason. When it is liquid it will follow every detail of the mould, so any deliberate features or defects will DEFINITELY show up.
A common mistake is to use unsealed MDF as formwork. MDF is nice and smooth but is unsealed (even if it is moisture-resistant) will suck moisture out of the concrete. Chipboard is the same. Use either melamine or sheet plastic.
If you’re making a really long or wide piece, never be tempted to butt two pieces of sheet material up together unless its plastic and you are extremely adept at joining plastic sheets (it can be done but is quite a skill).
If you do butt two pieces of melamine together for example, the joint will show up in your finished concrete and you won’t be able to hide, fill or sand that mirror of the joint out. You’re only option would be to go over the concrete with a microcement.
All corners and internal edges should be filled with silicone sealant and tooled off using silicone tools the ones in the video below;
If you are making any inserts to go in your mould which are not to remain in the concrete then you will need to ensure they are waterproof and smooth enough to be removed from the concrete without damage.
Polyester resin is a good and cheap option for going over the likes of MDF as is car body filler. Here is an image below of a sink mould insert made by one of our customers with our guidance.