The relationship with manufacturer is a two edged sword. Take Anand and Intel as an example. We get excellent info, but it tends to be just a bit bend - and its difficult to point exactly where the problem is. Its not intentional, but its difficult to avoid. But i think we can judge by ourselves and i prefer the early og good info.
For me it looks like gsmarena gets all their phones early. LG optimus G pro, is an recent example of that. And they have good more objective tools to compare the products. The photo comparison tool, is an excellent way to judge the phones. You should try it.
You reply made me look back at the s3 review, and i dont find anything suspicious today. Far from. When i read it back then i had the feeling, as you, it was a bit on the positive side. I dont get the same impression today. gsmarene was spot on then, and history proved them right. They tend to be very positive about all their phones/gear, but thats just their style. And i like it that way. I only find anandtech phonereviews better. And btw, the s3 review here, resembled a lot of gsmarena.
http://www.gsmarena.com/samsung_i9300_galaxy_s_iii-review-761p12.php
Regarding the s4, gsmarena was the first to rightfully critizize the s4 video in their review.
You can bash Samsung, but they dont generally get credit for what they do by the reviewers. That will probably change within the next year, but hopefully not so we end in the nonsense blind apple follower reviews.
Today we have people thinking the ip5 camera is the best in the world. If there is some company that manages to create stupid storries about their products its apple, and because of their size and marketing capacity, they get a lot of good reviews. Its only starting to stop, because people is getting outrigt bored.