Definitely better than any built-in deck amplifier.
Its 25x4 at 12.5v but most cars run at least 13.5-14.4v and at 14.4v it will put out 55x4. I'm guessing the actual output would be somewhere in between, but remember that this amp has its own power supply which is bigger and better than anything that will fit in a single DIN dash location.
For your speakers 50-75 watts would be your goal power rating. This amp might not win you trophies at your local sound off but it will give the power they are looking for. The amp also has a nice crossover (50Hz to 200Hz, 18dB/Octave). Turn it on for both front and rear channels and set it to high-pass. Turn the knob counter-clockwise all the way (50hz) and you'll be able to turn the amp up to louder listening levels without damaging your speakers or your ears. When you're ready for more bass, buy another amp and sub to complete that low end.