Some are going to tell you that the .25ACP is completely ineffective against anything but a squirrel yet these are the same people who would have no problem carrying a .22LR. Others will tell you that all a .25ACP will do is tick off their assailant. Still others will say that they would rather have a sharp stick...etc..etc.
Hogwash. Way back in the late 80's & up through the mid 90's, I carried a crappy Tanfoglio Titan and then a Beretta 21A both in .25ACP and it saved my butt on a couple of different occasions. Would I still carry one? No, because there are pistols like the LCP, P32, & P3AT at very affordable prices. Back in the day, the only option was the Seecamp and it costed an arm, leg, & first-born - all you had to do was wait a year to get it, too!
Any gun especially one that can fire off multiple shots is better than no gun. If a Raven in .25ACP is all you have, use it by all means. However, if and when you get enough money together for something in a larger caliber then go and get the larger caliber. In the meantime, shoot for the face/eye, throat, & crotch with your Raven -- Just make sure the thing will shoot w/o fail.