[HD2 Android Q&A, Help & Troubleshooting and Genera] baymon:
Here's a little benchmark that I did over my four different microsd cards:

-----------------------------------------------------------------------

CrystalDiskMark 3.0 (C) 2007-2010 hiyohiyo

Crystal Dew World : http://crystalmark.info/

-----------------------------------------------------------------------

* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

Sequential Read : 15.265 MB/s

Sequential Write : 11.265 MB/s

Random Read 512KB : 15.039 MB/s

Random Write 512KB : 2.124 MB/s

Random Read 4KB (QD=1) : 3.984 MB/s [ 972.6 IOPS]

Random Write 4KB (QD=1) : 0.021 MB/s [ 5.2 IOPS]

Random Read 4KB (QD=32) : 6.339 MB/s [ 1547.7 IOPS]

Random Write 4KB (QD=32) : 0.027 MB/s [ 6.5 IOPS]

Test : 100 MB [G: 76.5% (5941.9/7768.0 MB)] (x5)

Date : 2010/07/27 1:47:31

OS : Windows 7 [6.1 Build 7600] (x86) Class6 Card

Sequential Read : 20.772 MB/s

Sequential Write : 15.931 MB/s

Random Read 512KB : 20.217 MB/s

Random Write 512KB : 4.051 MB/s

Random Read 4KB (QD=1) : 4.652 MB/s [ 1135.7 IOPS]

Random Write 4KB (QD=1) : 0.041 MB/s [ 9.9 IOPS]

Random Read 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Random Write 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Test : 100 MB [G: 91.0% (7071.0/7773.0 MB)] (x5)

Date : 2010/07/27 2:04:14

OS : Windows 7 [6.1 Build 7600] (x86) Class 4 Card

Sequential Read : 20.828 MB/s

Sequential Write : 7.031 MB/s

Random Read 512KB : 20.597 MB/s

Random Write 512KB : 1.079 MB/s

Random Read 4KB (QD=1) : 4.648 MB/s [ 1134.8 IOPS]

Random Write 4KB (QD=1) : 0.038 MB/s [ 9.2 IOPS]

Random Read 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Random Write 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Test : 100 MB [G: 9.2% (1.4/14.9 GB)] (x5)

Date : 2010/07/27 2:32:08

OS : Windows 7 [6.1 Build 7600] (x86) Class 2 Card

Sequential Read : 9.637 MB/s

Sequential Write : 3.249 MB/s

Random Read 512KB : 8.770 MB/s

Random Write 512KB : 2.805 MB/s

Random Read 4KB (QD=1) : 0.900 MB/s [ 219.8 IOPS]

Random Write 4KB (QD=1) : 1.296 MB/s [ 316.5 IOPS]

Random Read 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Random Write 4KB (QD=32) : 0.000 MB/s [ 0.0 IOPS]

Test : 100 MB [G: 96.5% (14.3/14.8 GB)] (x5)

Date : 2010/07/27 2:19:04

OS : Windows 7 [6.1 Build 7600] (x86) Class 2 Card

The class 6 card was a transcend card that I just bought off amazon for about $20ish in hope to get a faster speed than my class 4 A-data card.

However, the benchmark says it all, the class 4 card is actually faster than the class 6 card on every performance benchmark, and it easily beats class 10 card with write/read speed of well over 10MB/s.

The 16GB class 2 card given by T-mobile HD2 is the worst performer. The other class 2 card is a Polaroid brand 16GB card.

So, I'd say that you should stick with the name brand on lower class (class 2/4/6) rating, as those big manufacturers tend to be more conservative in rating their class. I've seen a lot of people saying that their Sandisk class 2 card gives out a read/write speed of well over 20MB/s, that's class 20 for ya.

maybe everyone should try out the crystaldisk benchmark too.

*Sorry I forgot to say that I did the test using a microSD USB reader instead of mounting the card from the phone itself. I think there's a hardware limitation if you do so and you'll never achieve this kind of benchmark result, the typical read/write results that ppl have reported by mounting are around 2-3MB/s.