My husband and I BOTH love this t.v. show- which is a miracle! We have such different taste in sitcoms that we were amazed it appealed to both of us. With that being said I would say the best two seasons of this show are the 2nd and 3rd seasons- the humor is top notch and will have you laughing out loud each episode. I found that after the 3rd season things have gone done hill... beginning with last season and then sliding down more with this 5th season.
*SPOILER ALERT* Part of the reason I think this season wasn't as funny was b/c Robin and Barney date for a good chunk of it, which takes the most humorous part of the show- Barney and his womanizing ways- out of the picture. Some of the funnier episodes are the ones after their relationship is over. I'm all for thinking that Robin and Barney will end up together in the END... so hopefully they won't touch this area again until then!
*END SPOILER ALERT*
Since we own the first four seasons on DVD, I feel compelled to continue our collection by getting this one. All in all it's not a bad season... it's just not nearly as good as their first three seasons. It definitely helps move the story along of how Ted meets his wife though- and we actually get to learn more about her (finally!) So in that regard it's definitely worth the watch. Maybe Netflix it to see if you like it before you buy it though!