What do I need to know about it?

yes, stack your line with as much STR/TKL as you can

as long as no one passes on you with either the gun, trips, pro, I, box or bone.

Yeah you can with it if you score a lot of points on offense :)

Cannot be used as your base defense.

Can absolutely be your base defense !
I used it for years. Just recently changed over to try 4-3.
Won NCs with 5 2.

In my opinion any defense can be succesful if you have enough talent to make it work. I would think the 5-2 would be more difficult a defense to be very successful than the 4-3 or 3-4 but if you have the right players it can be done. My guess would be in the update it would be easier to exploti than it is now just as in real college football. However I would think it would be a good goalline defense.     

I just recruited the best dl I could find. Grew them by combo of playing time and starts.   I always had a great D with the 5-2; but of course for someone starting out or rebuilding a team you can't expect to start great.
          I have no statistical data to back this up but it seemed to me we could force turnovers in 5-2  more often than I could with other defenses and I have tried Every One as a baseline D ; even the Dime(I was curious . It's great vs passing but even with db with high tackling it still sucks vs run).

It's hard to recruit for the 5-2. Quality DLs are always hard to find, and now you have to have an extra one. OLBs are harder to find than ILBs, but now you don't need ILBs. I'm sure it's be successful if you had the right players, but I was never able to recruit the ones I needed.

I have been using the 5-2 defense for my Defience team for years and while I haven't won a NC I have played in it three times. you do have to recruit well but if you do you can be very competitive.

How do you defend the pass with that defense?  Does the extra DL give you that much extra pressure on the QB?

I've personally never played a 5-2 that could slow down the passing game out of the I or from Trips.

When you say its your "base" defense...what offensive formations do you use it against?  vs Box and Bone makes sense.  vs Pro Set and I also has a case.  Trips and Shotgun not so much. 

Currently I am not using it .  Using 4-3 . But with right personnel 5-2 worked fine as base D. 
        I am not sure why it works . It does give up high percentage of passes but not very many big plays . 
            However, I would recruit and grow excellent db's and starting outside lb's .  So it seemed to me the extra dl would  add pressure and my lb's and db's would get many turnovers.  Obviously, the better your db's and lb's the better it would work.
                    I ran 5-2 against everything except  Trips and Shotgun. Not 100 percent of the time though. But between 5-2 and Nickel  I ran little else.
                          Like anything in this game, the trick is Recruiting the best you can and Growing them the best you can. Otherwise , no matter what formations you run you will never get your team to the top.
                                  But I reiterate, if done right(and this applies to Any formations) , the 5-2 can win NC's.
                    

I use the 5-2 and have been relatively successful with it. You do need at least 2 defensive schemes though..... I wouldn't play 5-2 defense in an obvious passing down.....

i have no idea what works and what does not.  As far as I can tell, gameplanning is pointless