Honestly, the internal components are made by about three companies these days. The only difference is that housing and marketing as far as brands go.
While I agree with the first part, I highly disagree with the only difference being marketing and the housing.
When a wafer comes out of a fab, some are considered top quality, some flat out fail and then there are the middle quality units. Some fabs produce better silicon than others.
The "top end" brands like SanDisk, Lexar, Hoodman, etc are usually buying the grade A silicon, while the dinky 128meg "HP" branded cards for example are lower on the quality rung. In the past I've used nothing but Lexar CF cards, mainly because at the time they were the only one that supported Write Acceleration on the early Nikon bodies. I still have an use some of my earliest Lexar CF cards, specifically I have a 1gb card from ~2002 (that I paid over $400(!!!!) for) and it's still ticking along (and I know for fact that one has been washed at least once). I have a few "pro" SanDisk cards that I've also not had any issues with.
That said, I have numerous other non-top end cards that I've had issues with. I never used them for shooting, just for random things, transferring files between co-workers laptops, things like that. I've had two cheap "basic blue" SanDisk's, an HP and a Kodak card all go bad on me this year alone. My future-mother-in-law went to Mexico with her husband last year and had her 2gb Kodak card die on her on the last day there, losing all of her pictures.
Basically what I'm getting at is, not all cards are created equal in quality and I'm not even factoring speed in. Memory is definitely one area to spend the extra few bucks. It is after all, your images final destination. If it fails, it doesn't matter if you spent $8k on a top end Nikon D3S with a 24-70 /2.8, those images are gone.
Here is an example of what I'm talking about;
http://www.newegg.com/Product/Produ...yMark=False&IsFeedbackTab=true#scrollFullInfo
ADATA is what I would consider a mid-range brand. Not bottom of the barrel, but certainly not the level of a pro Lexar card. For those that don't want to click the link, I'll quote something of interest;
I have purchased 6 of these. Of those 6, I've had 3 fail for the same reason
A 50% failure rate! Out of 70 people reviewing that particular card, 34% of them gave it a 1 out of 5 rating.
With all of that out of the way, as far as what type of card to buy, that depends on what you're shooting.
If you're shooting with a low end P&S, there is no reason to spend the extra cash on a Class 10 or UHS card. Simply put, the buffer in the camera can't transfer it as fast as the card is. Same goes for flash-based camcorders. I believe the current max for camcorders using 1080p AVC HD is 24mbps, which comes out to 3MB\sec. That said, an honestly rated Class 4 card is really all you need as it should be able to write at 4MB\sec (a Class 6 is 6MB\sec, Class 10 is 10MB\sec). There are reviews out there that XYZ's Class 4 card couldn't keep up with my HD camcorder, simply because the manufacture stretched the truth a bit. You might want to get a Class 6 to be sure you don't have issues.
If you shoot nothing but landscapes, you don't need the fastest thing out there, you're generally spending minutes composing a shot rather than clicking the shutter as fast as you can. Shooting sports? Yeah, you want a fast card (*though again, if the camera can't write to the card at 10MB\sec, it doesn't much make sense to spend the money on a Class 10 card, does it?)
There are also "x" rated cards out there. The initial batch of cards I bought with my D7000 were Lexar Professional 133x. These are still "Class 10" rated, however they will actually transfer at 20MB\sec, twice as fast as the Class 10 spec. The SDA hasn't approved anything beyond a Class 10 spec, so manufactures will often add in the "old" method of classifying speed to separate their product for another.
Lastly, if you happen to have a D7000 and need to shoot fast, I highly recommend picking up the SanDisk Extreme Pro UHS-1 cards. They are by far the fastest out there. With the latest 95MB\sec version I can shoot basically non-stop in RAW without ever having to wait for the buffer to clear. They are the only card that I've found that can keep pace with my older CF based Extreme Pro 600x's. Do keep in mind, unless your camera specifically supports UHS-1, it will act like any other Class 10 card (though you will be able to download your pics onto your PC 9 times faster). Currently the only two bodies that I'm aware of that support UHS-1 is the Nikon D7000 and the Sony A77. Also keep in mind, the latest UHS-1 cards from Sandisk are 95MB\sec, the older versions were 45MB\sec.
Hope that helps!