I used to work in the workshop for Duncan & Todd, Scotland only, but every optician is the same.
There is two kinds of reaction lenses, Kodak Sunsensors and Transitions. Both do the same but the Kodak ones are not as dark as the Transitions.
I personally haven't wore them but there is a few different factors you need to think about.
Depending on your RX can you get them. If it's straight forward, then it won't be a problem. If its too high the lense might not go high enough to take your RX, different materials are used for higher indexed lenses.
The cost, can you afford it? These lenses can scratch easily, so if you do drop them or whatever can you afford to get them replaced.
Frames, you can only put certain lenses in to certain frames depending on there design. You might see some nice designer frames, but there is a curve on the frame and your RX is not high/low enough to encorpoate the curve, new choice of frame.
x4biddenxlustx - if it's just single vision -3.00, then you won't have a problem having new lenses fitted to a old frame. You can get new transitions, or if you can't budget for them, then depending on how good a condition the lenses you have at the moment then you can get them tinted, if you have a second pair also.
hth's