Always been a fan of stock alloys and rims, but an offer I could not refuse came along (cost ~10k), and I recently installed some used after market alloys on the car.
True to previous experience, the wheels started wobbling at high speeds. Again, kind of knew that the hub centering was off. Ran around with it for a long time, laziness and lack of time contributing to postponing that trip to a friendly tire balancing shop.
Some reading on the issue - http://en.wikipedia.org/wiki/Wheel_sizing
The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Some factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric. Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to center the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down. Centerbore on aftermarket wheels must be equal to or greater than that of the hub, otherwise the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock or slide into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub. These adapters are usually made of plastic but also in aluminum.
More reading - Why Hub Centric Rings? - HubCentric-Rings.com
Good shops have the rings of different sizes ready made, and I went to one. Fitting involves sanding down the hub to remove the rust first, and very carefully inserting the aluminum ring. Tap with a soft hammer evenly to make it go in fully. The ring is pretty fragile, and is made of some low quality aluminum. According to the shop owner, making it fragile will allow for easy removal later. A steel ring would kind of get "stuck", it seems. Picture below
Hub ring on the front wheel -
And some random shots from different angles -