timing advancers

[b1k3rdude]

I have a cheap ebay 4 degree advancer fitted, it improves low to mid range response and helps with starting both cold and warm, the offset is that what it gives at the low end it takes away at the top. It's swings and roundabouts really but if you spend more time in the lower half of the rev range than the top end you might benefit from fitting one.

Ive had a proper plate but never fitted it, its all sound good - but can someone remind me what the downsides are again..?