Here is some more info about serums for those who haven't had much experience with them yet.
Most of the cryo boxes you get in game (other than the ones you buy from black market) have one level 1 serum in them. There are 8 different types of serums, one for each unit type's offense and defense, and each box gives you a random type. To get higher level serums you have to combine 2 of the same type and level to get one that is a level higher. So if you want to make a high level serum, here are the amounts of level 1 serums (all of the same type) you would need:
level 2: 2
level 3: 4
level 4: 8
level 5: 16
level 6: 32
level 7: 64
level 8: 128
level 9: 256
level 10: 512
level 11: 1024
level 12: 2048
Keep in mind that each box gives a random serum type, so your chance of getting the type you want is 1 in 8. *All estimates from this point on assume there is an even distribution between the 8 types, although I haven't done enough testing to confirm this it seems to be a reasonable assumption.* This means that to make a level 12 serum of a specific type would require you to open (2048*8 ) = 16384 boxes. You can buy higher boxes in black market that give up to 2 level 5 serums, for 3300 crystals each. To create a level 12 serum from these would take 128 level 5 serums, or (128/2*8 )= 512 boxes, which would cost 1.7 million crystals. This is an optimistic estimate, in my experience they usually gave one level 5 and one level 3, while this assumes 2 level 5s every time.
Once you activate a serum, you have to pay crystals to remove it. This costs 500 crystals for level 1-4 serums, 2500 for level 5-8 serums, and 5000 for level 9-12 serums. This means that it's best to not activate a serum until you have reached the final level you want to get it to, but if you have already activated one at level 3 or higher it's better to extract it than destroy it.
Here are some examples of what the different levels of serum do:



They seem to increase in power at a steady rate, so if a level 1 serum gives a type of unit +1% power, a level 2 would give 2% and a level 12 would give 12%. These numbers are added to other bonuses and not stacked. So if you had a trooper at max research level with a +5% damage artifact and +12% from a serum, it would have 40 *(1+.4+.05+.12) offense power, NOT 40*1.4*1.05*1.12. This applies to defense boosts also. The defense boosting serums also have the interesting feature that they give different boosts against different units. for example the ones that boost the defense of air units have a higher boost % vs infantry than vs other air units or artillery. They seem to help "even out" units, by giving them higher boosts against things that they are normally weak against. The ones for cheaper units ( infantry) also seem to give a higher boost for each level than the ones for artillery or armor. This means they can be useful for the units that give you the cheapest power (for example troopers). Get 3 level 5 infantry offense boost serums, 2 +5% perm infantry offense artifacts (these are fairly easy to get if you do battles a lot) and max trooper research level, and your troopers will each have 40*(1+.4+.15+.1)=66 offense power. With level 12 serums this would go up to 75 offense (although it would also cost a prohibitive amount of crystals).

In conclusion, serums can be either a helpful advantage or a giant money pit, depending on how you use them. I would recommend not activating serums until you reach at least level 5-6. But don't hold off for level 12, unless you plan on playing td until the day you die (or spending your entire life savings on it). Remember each level takes twice as long/much money to reach as the level before it. You can only activate 3 serums at a time, so you will have to decide which units you prefer to use (or just go with whatever highest level serums you get). Is 8% extra offense on artillery better than 4% on artillery and 4% on armor? It all depends what type of units you use.         

~ Biohazard