I generally like an anime better than its manga counterpart, even if the manga is the original one most of the times. Dunno why, but it's harder for me for no apparent reason to understand what's going on reading the mangfa (that's why I'm watching Another even though I read the manga, I thought it was awesome, but I didn't understand some of the things).
ehhh I hear from my friends that Durarara is really good. However I watched the first episode and I can't say I'm compelled to continue watching it. It just altogether doesn't seem interesting enough and I feel like my time is better used reading other good manga. I think as a general rule the manga is better than the anime. Of course the manga can't compete with the anime fight scenes like Last Airbender or Naruto, but the anime on the whole tells the story better and is not filled up with stupid fillers like the anime has.
I hate fillers.
Anyways, I don't see why Durarara is so good. eh. Feel free to disagree with me
As for the manga versus anime thing, first, my rule of thumb: if both the manga and anime are out, watch the anime first. Here's why: First, animes have sounds and motions and are generally more engaging. They also cost more to produce, so they're generally more polished visually. Animes often leave a better, or at least bigger, first impression than mangas do. Watching Kenshin, Bleach, Hitman Reborn, etc. first gave me more to go on when I went back to read the mangas for them. But there's one thing you can't do for animes and that's control your own pacing. When you're reading manga, you can skim pages that you've already seen the anime and know the plot to. You can use the manga to fill in any gaps/plot hole problems, or just enjoy your favorite parts again. If you read the manga first, you can't exactly fast forward though an anime. Well you can, but fast forwarding, especially on a computer is super annoying, so you might as well save yourself the hassle and just not watch the anime. I read Bakuman first because it didn't have an anime when I first started it. When the anime came out, I tried watching it because I liked the manga, but I was so bored I used it as background noise for a couple episodes and then wound up just giving up. You're also more adaptable to fillers because if you don't know what originally happened in the manga, you're not as certain that they're actually fillers and more willing to just go with the flow of the anime. You also won't have voice actor headcannons interfering with your enjoyment of the anime.
As for fillers, there are enjoyable fillers and not enjoyable fillers and I don't think it's fair to paint them all in the same brush. Most of Darker Than Black, even the first season, is filler. The original manga is almost completely different (and worse, in my opinion), but the Darker Than Black anime is brilliant, one of the best shows I've seen. Season 1 of Shugo Chara had some cute fillers that I didn't mind. The D.Gray-Man fillers make the show a bit more balanced and not so crushingly depressing. And Gintama fillers, well, it's Gintama. Good luck telling one thing from another.
But I agree that fillers can also ruin a show. Seasons 2 and 3 of Shugo Chara, bleh. Magi was just a complete mess by the end of it. (I'm scared to see what they're doing for season 2) Fullmetal Alchemist, while not terrible, doesn't come close to comparing to Fullmetal Alchemist: Brotherhood.
Basically, everything has an exception. If you haven't found an exception, you just haven't watched/read enough stuff yet.
As for Durarara, it was a visual novel first, so if you actually want to have a good experience, try to find the visual novel. And as for the first episode thing, don't give up? I feel like the pot calling the kettle black, because I watched the first episode three times without starting the show before someone finally convinced me that it does indeed get better. The first episode of Durarara throws way too many characters and partial facts at you without telling you why any of it actually matters, but you'll get a lot of explanations and actual plot if you just stick with it. And I'll say this, none of the characters are who you think they are.