I have only been using the arduino for a short time, a couple of months now and I have learnt so much I wish I had tried it a long time ago.
I have a background in electronics, but not in designing projects, mainly just faultfinding & repairing, but I finally bit the bullet and ordered one, 3 days later I ordered another. Amazing product.
I love the arduino because it is simple, yet can be very powerful, can be tiny, but can be expanded into a very large project. The platform is easily programmable with easy access to all pinouts and a huge library of knowledge, both in the playground, reference section and in the forums. All of the knowledge and samples are free, so the only cost is the parts for your project.
The community is fantastic, I am constantly amazed at all the ideas that people are trying out, from the very simple, like automating lights, to complex systems interacting with many sensors. I feel that anything I would ask on the forums has either been answered and just a search away or someone will pop up with the answer very quickly.
So far some of the projects I have been working on are a laser triggered camera trigger for capturing water droplets hitting water, a telescope alt-az mount for a friend and my home automation system which interfaces with my mobile phone by using a 433mhz RF transmitter with home easy wireless sockets, lights and a door interface.
I have done many smaller project, mainly just as proof of consept, that will later be implemented into other projects. It has sparked alot of imagination and when I see something I always think "Ooh, I could automate that with an arduino"
I will soon be starting out on fabricating my own custom pcb instead of using the arduino prototyping boards for a more permanent solution and moving the prototyping boards onto other projects.
My next purchase will be an arduino mega for the bigger projects that I have in mind.