Yes it's worth it and easily the most valuable credit card reward you can earn if you use it effectively. The best value you will get from it is booking long haul business class flights as this can save you hundreds of thousands of avios. For example, I recently booked return business class flights to Buenos Aires for 220,000 avios + £1,200 for 2 people which would have cost 440,000 avios + £1,200 had I booked it without the companion voucher. If you value 1 avios at 1p which is generally regarded as a good rule of thumb (although recently you have been able to purchase avios for a low as 0.84p), that works out at £1,700 per ticket which is a pretty decent price for 2 16hr flights in business when comparable economy tickets are costing £1,200 or so. If you compare it to a cash fare of a business class ticket (£4k + for the same flight I am referring to) then it represents even better value although if you would never pay that amount for a cash fare (as I wouldn't) it's not a great comparison.
In contrast you can end up getting pretty poor value from the voucher in you redeem in long haul economy. Taking an example of New York, an off peak return economy fare for 1 person is 50,000 + £100 for one person, so if you use your voucher for two tickets you will be paying 50,000 + £200 or £350/ticket using the 1p avios valuation. It's quite easy to find flights to New York for a similar price and you wouldn't have forked out the £250 fee for the Amex card to earn the voucher.
Redeeming short haul economy you aren't saving much avios so again you're not getting great value from the voucher considering the £250 fee to earn it.
So it does take some research to understand where to get value and you also need a decent avios balance to utilise it effectively.
In terms of getting the flights you want, yes unfortunately this isn't straightforward. For business class redemptions BA only release a guaranteed 4 seats on each flight 355 days before the depature date. For very popular destinations (e.g. Sydney, Singapore, Maldives, Japan), these get snapped up within minutes. BA release more seats sporadically depending on how well cash tickets are selling on each flight.
You therefore need to plan a year ahead when booking flights or be flexible in terms of dates/destination when redeeming the voucher to get good value. You can use tools such as Seatspy to track the release of more avios seats.
The above is just the tip of the iceberg. I'd suggest reading through the guides on
https://www.headforpoints.com/
