I decided to try btrfs on F13 (2.6.33.6-147.2.4.fc13.x86_64 kernel) with the 
following fs_mark command and a 1.5 TB Seagate S-ATA disk:

# fs_mark -s 0 -S 0 -D 1000 -n 1000000 -L 1000 -d /test/ -l btrfs_log.txt

btrfs starts off at a fantastic rate - roughly 3-4 times the speed of ext4:

FSUse%        Count         Size    Files/sec     App Overhead
      0      1000000            0      20815.9          6257344
      0      2000000            0      17531.2          6310061
      0      3000000            0      10656.4          6473972
      0      4000000            0      17933.6          6291377
      0      5000000            0       6687.9          6708750
      0      6000000            0       6494.3          6792701
      0      7000000            0      18211.1          6266870
      0      8000000            0      18518.4          6231522


Then it chugs along, but hits relatively long periods of slowness:

     20    231000000            0      16991.8          6216216
     20    232000000            0      17524.1          6290540
     20    233000000            0      17088.5          6456494
     20    234000000            0       1259.1         10185066
     20    235000000            0       1419.0          8144068
     20    236000000            0       1670.9         12154381
     20    237000000            0       1863.6          8582337
     20    238000000            0        929.1          9714860
     21    239000000            0       4100.7          7385278
     21    240000000            0       3486.6          8773568
     21    241000000            0       2224.7          7320514
     21    242000000            0       3761.8          7166617
     21    243000000            0       3197.0          6918920
     21    244000000            0       1293.1          8486926
     21    245000000            0       3189.7          7396151
     21    246000000            0       3164.7          7103912
     21    247000000            0       4028.8          6770544
     21    248000000            0       3698.7          7133084
     21    249000000            0       2247.9          7421000
     22    250000000            0       1579.3          9833236

I will fill this and get some plots, etc to do relative performance & then 
wanted to try the current RC kernel just to refresh.

Ric

(Whole log file appended)