Active subwoofers have long been seen as the simplest solution when it comes to adding that extra punch to any sound system. Buying an active enclosure eliminates the need to start matching subwoofers with amps, designing the box to suit the subwoofer and connecting them up in a tidy fashion. To the audio novice this can become a very complex and difficult process, which is why active enclosures are so good when building your first system.
The value for money on the active kits is extremely good. The subwoofer is primed to perform to its fullest straight from the box, companies put millions into the development of the right enclosure and this really shows. Product lines such as the Fusion Encounter series and the Vibe Black Air series are both excellent examples of what can be achieved with an active enclosure; this is probably why they are the market leaders.
In this market you get what you pay for and from my experience go for the better quality items. The slim fitting units that slot under seats really can't cut it for volume and SQ against the full size enclosures. These are the more expensive units and for the best quality you are going to be looking between 150 200, this still works out cheaper than building a custom system.
The active enclosures are excellent quality kits, yes you can build a better system by going custom, but you will pay a lot more in order to achieve this. For value/performance there really is no better option out there.