how much difference does work experience makes in interview level of CAT exam,especially in iims?

More than experience, its the relevant experience that matters or I shoukd say how do you connect your experience with mba to build a future. For a fresher same holds true, that how do you link your past, future plans with mba and justify it.

Every college of IIM have their own way of giving weightage based on your job experience.

IIM-C

Months.  Points

<6.                0
6-9.              0.5
10-12.          1
13-15.          2
16-18.          3
19-21.          4
22-24.          5
25-27.          6
28-30.         7
31-36.         8
37-39.         7
40-42.         6
>=43.          5

IIM-K

Months      score
0-5.              0
6-12.           0.5
13-15.         1.5
16-18.         2.5
19-21.        3.5
22-24.         5
25-27.        3.5
28-30.        2.5
31-36.        1.5
>36.            1

IIM-Raipur

Months.    Score
<6.                0
7-12.            2
13-18.          3
19-24.         4
25-36.         6
37-48.        3
>=49.        0

IIM-RANCHI

Months.     Score
<6.                 0
6-11.              1
12-17.           3
18-23.           5
24-29.           4
30-35.           3
36>.               2

IIM ROHTAK

Months.    Score
<12.             0
12-23.        40
24-35.        60
36-47.        20
48-59.         10
>= 60.          0

Similarly other IIMs also have their way of evaluation,but if you're a fresher,work experience will cast no effect on your profile,unless and until you've a full time working experience with a company/firm.
Note : Internship is not considered as work experience.

There are two benefits of having work ex. First, during interview, most of the questions were revolving around your work experience, if you have decent months of work ex, say 2 years. If you have prepared well about your work then you have done almost 70% of your interview preparation.
Secondly, most of the iims give weightage to work ex students. A work ex of 30 months is given the highest point in most of the iims. So, if you have work ex more than 2 years then you have high probability of getting selected than those who are freshers.