Here is what I would say on this subject. The first thing to look at is the brisket itself. I am assuming that you bought a whole or half beef from a meat processor in your area, if that is the case then you have a good piece of meat to start with. If on the other hand you bought it at a store then first check to see if it has been injected already. If it has, federal labeling requirements mandate that the packer label is as follows (or something similar) injected with a 15% solution of _________. Usually what they inject is water, salt, and phosphates of some kind.
If the beef is from the store, you can still inject flavor but be careful how much sodium you have in the flavor, it may get salty. The phosphates will help keep the beef moist during/after cooking and will also help with some tenderness. Its not a bad thing other than you are paying for someone else's water!
Now, if you have fresh beef that hasn't been injected you can have more fun. Injecting is always a good idea in my opinion, as you are adding moisture to the meat along with putting flavor in the muscle tissue as well as on the outside. What you inject is another fun thing. I usually find a marinade or injectable seasoning that has some phosphate in it so that the moisture and seasoning will stay in the meat a bit better. What flavors you choose is totally up to your pallet and imagination. I have a marinade that I use called Pa's Black Bull BBQ that has a sweet and spicy taste to it. It also has phosphate in it. The marinade has enough sugar/dextrose that it glazes over during cooking and creates a nice crust and some of the best burnt ends I've ever had. I also know of people that inject a Butter Flavored seasoning into the meat and do different topical rubs. If you think it will taste good give it a try.
As far as the injecting of the meat goes, you probably won't get more than a 20% pump. Meaning the weight of the marinade you inject will be about 20% of the weight of the meat. This may not seem like much but trust me its enough! Make sure to use an injector that has multiple holes on the needle, this will help with a more even distribution of the marinade. Just make sure that you inject all parts of the meat and as evenly as you can, watch the muscles as you inject and they will plump up. Its fun to watch! Go from both sides of the meat as well as from the ends to get it evenly pumped.
At this point you can rub on a seasoning and go straight to the oven/smoker or let it sit and soak for a while, your choice. eman is correct, don't re-use the liquid that runs out of the meat, consider it contaminated and dispose of it.
Enjoy, there is nothing quite like a good brisket. In fact, we have some in the smoker right now.