I could add a whole chapter on this, but I agree with everyone else here. I have a lot of carbide that has fallen in my lap. They are great but not absolutely necessary. HSS tends to be sharper on average, although sharp carbide endmills do exist if you know what you are looking for (see link below).
Tool holding rigidity is a huge deal. Use the shortest length flute you can for the cut you want to take, and stick the endmill out the least you can without your spindle hitting the work, even if it is harder to see what you are doing.
I have found that a lot of my "all-purpose" endmills don't do very well in aluminum and can raise a burr. These are my favorite for aluminum, brass, copper, plastic, etc. Don't try them on steel, they won't last long.
3 flute and 45-degree helix allow harmonic balance at high speed condition and smooth cutting. Mirror face excellent surface finish & superior chip evacuation. High velocity milling of Aluminum and other non-ferrous materials.
www.ebay.com
Steel, stainless, and cast iron typically don't have issues with chip packing in the flutes so more flutes is better. Softer materials will have issues so 2-3 flutes is ideal.
I am happy to buy any import lathe tool, however rotating milling cutters require more care in manufacturing than fixed tooling. I've found the cheap endmills to not be as reliable as domestic tooling.
Don't spend a ton of money to start. You will crash and break cutters much faster than wearing them out. Better to do it with the cheap stuff at first.
Buy good quality collets or tool holders. The concentricity (runout) makes a huge deal when working with cutters under 1/4".
