I have one stick Micron and two sticks generic (yuk) and they run fine. However, you will be limited, as always by the capabilities of the slowest RAM. For example - if you had a stick of Micron and a tsick of Corsair (both PC2100), and the Micron could cope with speeds of 290MHz, but the Corsair could do 310MHz, the overall limit would be 290MHz, the slower one.
Same goes for mixing speeds. A stick of PC2400 and PC2700 in a rig will have PC2400 as the limit to how far you can push the memory.
If possible, stick with one brand, but you should be OK with mixed.