It was quite enjoyable in many moments. However, as the series went on I liked Dawn less and less (unlike the majority of people who I've noticed seem to go the other way). But then again, part of that was my problem with Piplup. I mean, is there any need to add a Bubblebeam with Pikachu's Thunderbolt to blast off Team Rocket? Thunderbolt was an over-used cliché anyway, but at least it looked cool.
Yes, there were way too many fillers, however some of them were good (the pointless release of Ambipom did at least make for an entertaining episode).
Other characters were generally good too. Khoury and Lyra were quite good, particularly when his Totodile and a wild Gible were smashing bricks on their heads - I actually feel off my chair with that one, I'm slightly ashamed to say. Paul was cool, a good contrast with Barry, who would've become rapidly annoying if he hadn't been excluded for so long. The whole "hey, it's Paul" scene every time they saw him was annoying, but meh - Brandon's quite obviously his relative, did you see how similar they acted? Brandon's been overcome with rage too, when he got angry with Ash in Battle Frontier, just like Paul whilst battling Brandon.
Chimchar's storyline was much better than Charizard's, and the higher quality of animation meant that I actually preferred Infernape (although if Charizard ever returns for another battling with a dragon / legendary, I will definitely be happy), Torterra was tragic, but another pokémon I liked, so who cares. In general, I liked Ash's pokémon, even Staraptor, who was luckily a lot different from Swellow.
I also thought the league was handled very well. Not only did Ash bring in more pokémon (alike to the Silver Conference), he also defeated his rival in an epic battle that, for once, was actually entertaining in all three episodes. The addition of new techniques, and much more focus on game mechanics such as Trick Room, Toxic Spikes and unique moves such as Sleep Talk (Heracross would've taken out Fantina in five minutes) made it all the more interesting. Tobias was a good move, because honestly, the calibre of Ash's pokémon, such as Sceptile, Infernape, Pikachu and Gible (his Draco Meteor would've probably felled Latios if he hadn't been taken out by Darkrai), meant that nothing other than a team of legendaries could defeat him without much disappointment and some n00bishness on Ash's behalf. The fact that Sceptile, which along with Infernape and Gible is one of my favourite pokémon ever to appear in the anime, was shown at his best was the icing on top of the cake for me. Parallels with the Silver Conference were good, i.e. Infernape overheating the battlefield, and the defeat of a rival - not so much for nostalgia, as I only really saw the end of some Johto episodes, not league ones, but more for it being the right formula.
Did I mention the quality of the animation? Particularly in Sinnoh League Victors, where it looked so much clearer than ever before, this was brilliant. Advance was good, a major step up from Johto and Kanto, but SLV and onwards is brilliant. Little details such as that white backgrounded small-character bit between Infernape and Electivire were just brilliant, a lot more effort than is really needed for what is, essentially, a children's t.v. show aimed at advertising the games.
Jesus, this is a large post. Sorry about the wall of text. Conclusively, whilst the amount of fillers and contests (the announcer lady's voice irritates me) were obscene, overall the quality of everything was quite good, and as it's the only saga I've watched as it happened (excluding a catch-up of the whole first Diamond and Pearl series), and it was thoroughly enjoyable to watch. So much that, in the end, I decided to watch Black and White after all :)