I definitely think it's worth it.
Harry's music is incredibly beautiful and for the most part, it stands on its own. There are certain tracks that work better in the film than on their own, like "The White Witch" and "The Stone Table", but they are few and overall, it's an absolutely fantastic score, magic.
Things to consider:
There are at least three different releases available, two official and one not so official. You have the standard Original Motion Picture Score, which at least originally came with a lenticular 3D cover insert, you have the 2 disc Special Edition Soundtrack that comes with a DVD that has a "CONCEPT ART GALLERY" that contain a couple of pieces not found elsewhere + beautiful fold out packaging and then you have the non-official "The Recording Sessions" release . The Recording Sessions CD comes with two discs containing most, but far from all of the score recorded for the film.
Here's the rundown:
Original Sountrack (Music Composed by Harry Gregson-Williams)
Content:
1. The Blitz, 1940
SCORE (2:32)
2. Evacuating London
SCORE (3:38)
3. The Wardrobe
SCORE (2:54)
4. Lucy Meets Mr. Tumnus
SCORE (4:10)
5. A Narnia Lullaby
SCORE (1:12)
6. The White Witch
SCORE (5:30)
7. From Western Woods to Beaversdam
SCORE (3:34)
8. Father Christmas
SCORE (3:20)
9. To Aslan's Camp
SCORE (3:12)
10. Knighting Peter
SCORE (3:48)
11. The Stone Table
SCORE (8:06)
12. The Battle
SCORE (7:08)
13. Only the Beginning of the Adventure
SCORE (5:48)
14. Can't Take it In
PERFORMED BY IMOGEN HEAP (4:58)
15. Wunderkind
PERFORMED BY ALANIS MORISSETTE (5:35)
16. Winter Light
PERFORMED BY TIM FINN (4:29)
17. Where*
PERFORMED BY LISBETH SCOTT (2:10)
* Bonus Track / Not Featured in Film
Special Edition Soundtrack (Music Composed by Harry Gregson-Williams)

Content:
DISC 1: Audio CD1. THE BLITZ, 1940
SCORE (2:32)
2. EVACUATING LONDON
SCORE (3:38)
3. THE WARDROBE
SCORE (2:54)
4. LUCY MEETS MR. TUMNUS
SCORE (4:10)
5. A NARNIA LULLABY
SCORE (1:12)
6. THE WHITE WITCH
SCORE (5:30)
7. FROM WESTERN WOODS TO BEAVERSDAM
SCORE (3:34)
8. FATHER CHRISTMAS
SCORE (3:20)
9. TO ASLAN'S CAMP
SCORE (3:12)
10. KNIGHTING PETER
SCORE (3:48)
11. THE STONE TABLE
SCORE (8:06)
12. THE BATTLE
SCORE (7:08)
13. ONLY THE BEGINNING OF THE ADVENTURE
SCORE (5:48)
14. CAN'T TAKE IT IN
PERFORMED BY IMOGEN HEAP (4:58)
15. WUNDERKIND
PERFORMED BY ALANIS MORISSETTE (5:35)
16. WINTER LIGHT
PERFORMED BY TIM FINN (4:29)
17. WHERE*
PERFORMED BY LISBETH SCOTT (2:10)
DISC 2: VIDEO DVDFILM ART GALLERY
CONCEPT ART GALLERY
BEHIND THE MAGIC OF NARNIA
FEAUTURING THE SCORE WITH
HARRY GREGSON-WILLIAMSTHEATRICAL TRAILER
MUSIC INSPIRED BY THE FILM (TRAILER)
34 PAGE COLLECTIBLE BOOKLET WITH FULL COLOUR PHOTOS AND ARTWORK CONTAINING INFORMATION ABOUT THE FILM AND LYRICS FOR ALL THE SONGS.
THE RECORDING SESSIONS 2-DISC SET, UNOFFICIAL/BOOTLEG (MUSIC COMPOSED BY HARRY GREGSON-WILLIAMS)
Content: (Tracklist below includes tracks from different sources, but most are available on the 2-Disc set I bought on eBay. Cover art of this set is different, I created the above image in Photoshop for my Ipod Touch/iTunes)
1. THE BLITZ, 1940 (2:32)
2. EVACUATING LONDON PART I (2:28)*
3. EVACUATING LONDON PART II (1:14)*
4. MRS. McREADY (0:51)
5. THE WARDROBE (3:26)*
6. LUCY MEETS MR. TUMNUS (4:25)*
7. A NARNIA LULLABY (1:20)
8. THE WHITE WITCH (4:39)
9. CONFUSED (1:11)
10. JUST YOUR IMAGINATION (1:20)
11. PETER, WAKE UP! (5:49)
12. THE ADVENTURE BEGINS (0:51)
13. THE WORLD OF NARNIA (5:49)
14. FROM WESTERN WOODS TO BEAVERSDAM PART I (2:19)*
15. BEAVER'S HOME (0:52)
16. THEY'RE AFTER US (1:10)
17. INSIDE (1:31)
18. DISCOVERED (0:53)
19. HIDE AND SEEK (2:23)
20. FATHER CHRISTMAS (3:24)
21. ACROSS THE MELTING RIVER (3:33)
22. TO ASLAN'S CAMP PART I & II (ALTERNATE I) (5:17)
23. TO ASLAN'S CAMP PART II (ALTERNATE II) (1:30)
24. KNIGHTING PETER PART I (2:04)
25. BROTHERS (1:06)
26. ASLAN AND THE WITCH (3:26)
27. THE STONE TABLE PART I (ALTERNATE) (7:25)
28. THE STONE TABLE PART II (ALTERNATE) (4:14)
29. YOU HAVE TO LEAD US (2:23)
30. THE BATTLE PART I (2:46)
31. SADLY (1:09)
32. THE BATTLE PART II (3:14)
33. PETER VS. THE WITCH (2:32)
34. THE ROYAL CORONATION (1:33)
35. ONLY THE BEGINNING OF THE ADVENTURE PART I (4:19)
36. ONLY THE BEGINNING OF THE ADVENTURE PART II (1:21)
37. CAN'T TAKE IT IN (4:58)
38. WUNDERKIND (5:35)
39. WINTER LIGHT (4:29)
40. WHERE (1:57)
The tracks on this album aren't in any kind of order at all, so I've tried to arrange them in chronological order, but there are a couple of tracks I haven't been able to figure out where to put:
38. PERPETUAL WINTER (1:16)
39. NARCISSISM (1:29)