Third party gear/accessories have been quite popular recently. I think its a must have for beginners, since they're new to photography and they just want something cheap and practical. Pros use these too, but seldomly. Don't get me wrong, third party brands are there for a reason - to provide a cheaper alternative than the already expensively-priced manufacturer's lenses. Take the Tamron 70-200mm f2.8 for an example. I'll compare it to Nikon's equivalent Nikkor 70-200mm f2.8. In terms of price, the Tamron is definitely cheaper which makes it a no-brainer. The Tammy(a nickname for Tamron's lenses) is priced around RM2,800 whereas the Nikon is priced at RM8,200. Yes, the numbers don't lie. The Tammy is about cheaper than the Nikon. Quality-wise, they're practically almost the same. Better yet, Tamron is said to be sharper than Nikon and its labelled Macro which allows it to get up close to get macro shots(not 1:1 reproduction ratio, sadly).
It actually depends on what type of consumer you are. Do you have a budget? If budget is your case, you're practically stuck with third party gear. Buying cheap third party gear isn't always bad. Like the Tamron 70-200mm f2.8, I'd rather buy that then the Nikon 70-200mm f2.8 which costs 3 times more.
Have you been influenced?
Most of the people that confront me, are usually influenced due to those 'pro photographers' that think they know what they're talking about. People like these can be influential people, so you should watch out and read reviews from other websites. There are a few websites that provide unbiased reviews and some of them are Digital Photography Review, Ken Rockwell, Camera Labs, Thom Hogan and lastly SLR Gear.
Have you tried them out?
And because they're influenced, they just assume whatever that comes out from the site is true. Come on, we're all human and we all have different perspectives and preferences. Just because the lens that you're eyeing on is just a little too big, you shouldn't tell people it's a lens that sucks. It's basically down to preference. For me, ergonomics play a huge role in buying gear because you have to feel comfortable when you're taking shots. At the end of the day, what's the most important part, is you're happy with what you have.
Don't let people confuse you with their opinions. My number one advice, is to go to your shop, try the lens out, make sure everything's okay and then compare with others around that price range. That way, you'll be pleased. Any lens that you're eyeing on that you're not sure of? Email me, I'll try my best to reply you the next day.