The power focus is IMO generally worth it, because 29 bp vs. 32 karma and 100,000 nuyen is a really sweet deal unless your GM throws cash at you like candy. It's also a BIG starting power boost, and it lets you do other "power now for power later" trades that will help you in the long run, because it gives you a large dose of immediate power.
In a game where the GM was very very free with nuyen, I might not take the power focus and save up in-game for a force 6 one... but in, say, missions that means waiting most of a character's career.
The other time its worth it, though, is to avoid gear redundancy. Synaptic Boosters is actually a good example:
Suppose you were going to buy Synaptic Boosters 2, but you really do want SB 3. If you buy SB 2 at chargen, then replace it later, that's 160,000 nuyen down the hole - that's a TON of cash. Instead, I might prefer to spend the 5 BP now for Restricted Gear, get Synaptic Boosters 3 that I will never, ever throw away, and cut a lot of other corners, then buy other stuff with earned nuyen. Of course this assumes you really DO want that SB 3 which most people I think don't.
I'm not the biggest fan of taking it for Muscle Toner just because Muscle Toner is not very expensive, all things considered. My standpoint is that to take it, I should be NOT buying an item that costs over 25000 Nuyen, and instead buying a better version that I will never, ever replace.