Expertise

And here is a nice table with how expertise rating is converted into expertise and how much the npc chance to dodge and parry is reduced with thanks to the guys from the EJ-forums

Expertise Values
|Rating--|--Points--|--Remainder--|--Percentage|
9---------------1------------0.1975-----------0.25%
16-------------2------------0.0738------------0.50%
25-------------3------------0.0497-----------0.75%
33-------------4------------0.0256-----------1.00%
41-------------5------------0.0015-----------1.25%
50-------------6------------0.0994-----------1.50%
58-------------7------------0.0753-----------1.75%
66-------------8------------0.0512-----------2.00%
74-------------9------------0.0271-----------2.25%
82-------------10----------0.0030-----------2.50%
91-------------11----------0.1009-----------2.75%
99-------------12----------0.0768-----------3.00%
107-----------13----------0.0527-----------3.25%
115-----------14----------0.0286-----------3.50%
123 -----------15----------0.0045-----------3.75%
132-----------16----------0.1024-----------4.00%
140 -----------17----------0.0783-----------4.25%
148-----------18----------0.0542-----------4.50%
156 -----------19----------0.0301-----------4.75%
164 -----------20----------0.0060-----------5.00%
173 -----------21----------0.1039-----------5.25%
181 -----------22----------0.0799-----------5.50%
189 -----------23----------0.0558-----------5.75%
197-----------24----------0.0317-----------6.00%
205-----------25----------0.0076-----------6.25%

Expertise Rating converts into Expertise skill at a rate of 8.1975 expertise rating:1 Expertise at level 80, and reduces the chance for your target to dodge and parry by 0.25% per point of Expertise. 214 Expertise Rating is required to achieve 6.50% dodge reduction, which is the maximum observed value of any NPC.

In the begining when I first started to do heroics with my shaman I didn't even know how much hit I needed, let alone what is the best thing afther hit, so I started to wander. I thought more about crit rating and atack power more and never looked to much at expertise due to the simple fact that you usualy stay on his "behind" not in front. The first thing that came in my mind was ofcourse the Elitist Jerks forums and I was a little surprised becouse there was no definitive answer for me, not for my curent knowledgge of theorycrafting and class mecanics. I still searched and I found what belive to be god answer for my question(does expertise have any value?). Again stonebaby from BIGHITBOX made a comprehensive statement regarding the usefullness of expertise in raids and I advise you go and read his Analysis: Expertise
For now I belive that expertise is the second stat an enhancement shaman needs to cap.
If you want to know more about the stat then an enhancement shaman needs to cap you can read here http://wowshamanui.blogspot.com/2009/03/hit-rating.html

Sources:
http://bighitbox.com/index.php/2009/02/analysis-expertise/
http://elitistjerks.com/f47/t20765-shaman_enhancement/#Expertise_Rating