Well, Gen 5 is certainly a step up from Gen 4, but I'm still not entirely sold. The only cool legendaries are Zekrom and Kyurem, the trios are really uninteresting, and once again there are just too many legendaries. None of the starters looks interesting or cool, and some of them look like they're supposed to be evolutions of previous Pokémon but aren't, like Bouffalant and Tauros, Alomomola and Luvdisc, and Sigilyph looking like some ulti-Unown. Then we have Gothitelle, which is based off of a human culture, and I really don't like Pokémon that are based on human cultures (this isn't the first time though, happened with Mr. Mime and Jynx too). Many of the Pokémon have what I call the "Snout-syndrome", where they all have the same snout, or atleast their head ends as a snout, goes for Snivy, Zoroark, Reshiram, Cobalion, and way more in this generation, and it always looks kind of stupid. Of course there are also the occassionally ugly ones like Garbodor, Jellicent, Conkeldurr, Darmanitan and such.
Now that was the nasty stuff, Gen 5 also has a lot of cool Pokémon too, Carracosta, Zekrom, Hydreigon and Gigalith are all among my favorite Pokémon ever made. Then there are Pokémon like Volcarona which has awesome origins to it's design, speaking of which, there are also a lot of awesome typings in this generation. We also have our occassional cool Pokémon like Chandelure, Galvantula, Scolipede and Golurk.
Design-wise where would I rank this generation? Over Gen IV and under Gen II, so that'll be: Gen IV<Gen V<Gen II<Gen III=Gen I. At least for me. Sadly for me it does have more downs than ups, sure it has awesome and fun typing to some of the Pokémon, but the majority just look very bad to me, random designs like Cryogonal just look odd, and as I mentioned earlier, the snout-syndrome is there too often, and looking at the legendaries for X and Y it shows no sign of dissappearing again (although it isn't so bad on Yveltal as on the X one). This is of course a subject that is 100% up to each individual's opinion, so nothing I say here is a fact, just my personal experience.
Worth pointing out is that the gameplay is heaps better than ever.
Also, just a minor pet peeve of mine, but the names are pretty odd for many of these Pokémon, and I still have a hard time remembering them, but that's just me.