I've used both. Derailleurs give a very positive, direct feel when you pedal. It's pleasant in the same way a classic sports car is. The downside is that they require minor tinkering to keep them in proper adjustment. For a lot of people, this is part of the pleasure of owning and riding a bike -- just as with the sports car.

Typically, on a derailleur bike, the chain is exposed to the elements (sand, salt) and your expensive clothing, and this can be a deal breaker for many. However, if you have a really hilly commute, derailleurs allow you to get a wide gear range at a low price, and this can be a deal maker.

It all depends on what you like. There's no "right" answer.

Paul