Comcast user here, I really can't complain - I get excellent speed up and down 98% of the time, hovering between 5 and 6 mb/s.
However, I do keep the 'puter up to date re: drivers, Java VM and such.
Opera 8.5, XP Home, AMD Athlon 64, 1GB PC3200 RAM
Another thing to remember if just certain sites are slow is that the routing servers out there might be clogged or have heavy traffic to negotiate. Many folks don't realize how many 'hops' a packet might have to make from the origination server to your browser, and any one of them can be a bottleneck. It's not ALWAYS right to blame your ISP.
Do a TRACE ROUTE to CadillacForums.and see how many hops you take. It's not uncommon for a packet to go through 20+ servers before you see it. Here's an example :
C:\>tracert
www.cadillacforums.com
Tracing route to cadillacforums.com [65.38.186.68]
over a maximum of 30 hops:
1 5 ms 7 ms 6 ms 10.127.0.1
2 10 ms 7 ms 6 ms ge-6-2-sr01.salem.nj.panjde.comcast.net [68.86.212.93]
3 8 ms 8 ms 8 ms ge-2-24-ur01.franklinvill.nj.panjde.comcast.net [68.86.210.129]
4 9 ms 17 ms 8 ms te-1-1-ur01.vineland.nj.panjde.comcast.net [68.86.210.114]
5 8 ms 10 ms 11 ms te-1-1-ur01.absecon.nj.panjde.comcast.net [68.86.210.118]
6 8 ms 10 ms 19 ms te-1-1-ur02.absecon.nj.panjde.comcast.net [68.86.210.122]
7 12 ms 8 ms 11 ms te-2-1-ar01.absecon.nj.panjde.comcast.net [68.86.210.126]
8 11 ms 9 ms 11 ms po10-ar01.audubon.nj.panjde.comcast.net [68.86.208.22]
9 10 ms 9 ms 18 ms 68.86.211.10
10 11 ms 14 ms 12 ms 12.118.114.13
11 18 ms 17 ms 14 ms tbr1-p012301.phlpa.ip.att.net [12.123.137.62]
12 14 ms 14 ms 18 ms tbr1-cl8.n54ny.ip.att.net [12.122.2.17]
13 13 ms 13 ms 14 ms 12.122.80.21
14 18 ms 23 ms 17 ms so-4-1.car3.NewYork1.Level3.net [209.244.160.129]
15 16 ms 17 ms 16 ms ae-1-54.bbr2.NewYork1.Level3.net [4.68.97.97]
16 58 ms 57 ms 60 ms ae-0-0.bbr1.Denver1.Level3.net [64.159.1.113]
17 56 ms 56 ms 58 ms 4.68.113.54
18 60 ms 57 ms 58 ms 4.79.80.14
19 59 ms 58 ms 57 ms core-b.v33.ge-4-5.Level3.edge3.data393.net [208.42.224.117]
20 58 ms 61 ms 58 ms 208-42-224-102.unknown.data393.net [208.42.224.102]
21 * * * Request timed out.
22 59 ms 59 ms 60 ms 65-38-186-68.unknown.data393.net [65.38.186.68]
Trace complete.
As you can see, I took 7 or 8 hops before I was even out of the state, and then I went from New York to Denver....