You should definitely consider off-brand lenses, and especially flashes.
Generally, however, you get what you pay for. Tamron’s $770 70-200 isn’t as good as the $2,500 name-brand 70-200 f/2.8 lenses. It’s not as sharp, it doesn’t focus as fast, it lacks image stabilization, and it’s not as durable. However, it will allow you to take professional portraits for under $1,000, and that’s a feature the name brands can’t offer.
Similarly, the $190 Yongnuo YN-568EX doesn’t have all the features of the top-end name-brand flashes, but it has the same flash output and does everything most amateur and even professional photographers require for about one-third of the price. Even if you insisted on spending $600 on flashes, I’d recommend most photographers buy three of the off-brand flashes rather than one of the name-brand flashes.
Tip: Particularly with Chinese manufacturers such as Yongnuo, you can get even lower prices than normal by purchasing them on eBay directly from China.
Throughout this guide, I’ll recommend third-party gear when it’s a better value than the name-brand gear.