Home gyms are great, you can work out even when you can't get to the other gym. The problem comes when you get bored doing your workout alone and after two or three weeks that gym in the garage is sitting idle and the space is waste. If you make it a gym go the extra step and make it a gym you can love, one that can motivate you, encourage you to use it. Add music and videos, make it bright colors and lots of light. call your friends and invite them to use your gym with you. Because trust me, you doing a treadmill or stairmaster all by your self with only your earbuds is going to get old really quick. Other than that, just park the car in your cosy little garage and go for a run in the park.
I converted my garage to a home office, complete with lighting,plants, colors,sound and computers and I use it all the time.