How much?! Okay, it might be £450 dearer than a standard option but it's a very different beast. For a start, the Tacx iMagic Fortius fully PC-controlled, connecting via USB from a control unit on the bars.
You can use the included software to do a pre- programmed session, or you have the option of VR and full video modes, where the resistance adjusts to match the terrain. Lots of famous climbs are available on DVD for about £25 each, just pop in the disc and you can be grinding up Mt Ventoux in the (relative) comfort of your living room.
The motor brake resistance unit is extraordinary, capable of massive resistance (up to 1kW) but giving a very lifelike riding feel - it'll simulate downhills too, and you can even get a steering frame if you want to go for the full Virtual Reality experience. Make no mistake: this is as good as turbo trainers get, a hugely impressive piece of kit capable of logging all your data so that you can track your training very effectively over time.
As to whether you need something this fully-featured, complex and expensive, well that's between you, your training regime and your wallet. I'd suggest that you need to be pretty serious (or at least pretty rich) for it to be a realistic option; you only get out what you put in, and you'll need to sit on the Fortius for much, much longer than most other turbos to get a decent return on your investment. There's no doubting the quality though.