The Coaster is a commuter train that runs between Oceanside and San Diego - basically oriented towards North County commuters who work in the downtown SD or Sorrento Valley areas. It does follow the same route as Amtrak - and stops at all stations served by the Surfliner along its route - but also serves additional stations not served by Amtrak (i.e. Encinitas, Carlsbad, Sorrento Valley).
As a commuter train, it is cheaper than Amtrak - a roundtrip from Oceanside to downtown San Diego is $11 compared to $34 on the Surfliner. Obviously, you can't go north of Oceanside on Coaster as you can with Amtrak (though there is also Metrolink commuter trains at Oceanside that will take you to LA, Orange County, and the Inland Empire). I've never been on the Coaster itself, but given that I've been on the Surfliner between OSD and SAN I will say that you get a great view of the ocean from north of Sorrento Valley all the way to Oceanside!
I'd say its a great ride (as is the Surfliner) - go for it if you have the opportunity. Take it down to downtown SD or one of the other stops along the route. Just don't try and do it this weekend - all rail service south of Oceanside has been suspended due to track work (Amtrak is doing bustitutions during the closure).