Just because its slower does not mean cannot use. It really depends on your needs and shooting styles.
If you shoot landscapes, use the slowest one also can.
Determine whether the bottleneck (the weakest link) is the write speed of the CF card, or the write speed on your camera body. Decide than if its necessary to get the highest speed one; or lifetime, or 1 year warranty will do ( in that order)
I still use my 1 gb Lexar 40x from my 300D days. My rapid shooting style has not required me to go to any higher speed CF card yet. If I were using 300D, 350D, or 10D, still I will buy those 40x cards again used 1-4gb from the people here.