Most were pretty bad, IMO. The only ones I liked we're Togekiss, Gallade, Porygon-Z and Magmortar (screw the haters, Magmortar master race).
I hope that if they're doing any other evos or prevos of older Pokemon other than Sylveon that they are of a higher quality than Gen 4's new evos. Gen 6's designs have me pretty confident in that so far.
What does a nosy pepper do? It gets Jalapeno business. Official Frank Zappa of Every Board
Ambipom - I'm indifferent. Mismagius - Although I've not used one, I would like to at some point. Honchkrow - Looks decent but I'm not planning on using one anytime soon. Weavile - Like. I like Sneasel but I like Weavile even more. It's fast and strong in-game. Magnezone - It looks great but to tell you the truth, I haven't used one properly yet so I don't know how good it is in battle. Lickilicky - It's okay. I've used one in Platinum but I don't think it was to powerful. Rhyperior - Slow and not too great looking, but it seems powerful and defensive. Tangrowth - Never used one. I'm interested in using at some point (same for all Pokemon) but I still prefer Tangela over it. Electivire - Cool Pokemon but I don't think it Electabuzz needed an evolution. Same with Magmar. Magmortar - Look at Electivire. Togekiss - Haven't used one properly but I like Togepi more than Togetic and Togekiss. Yanmega - Yanma was great but Yanma is better. Leafeon - I like all the Eeveelutions (including Sylveon). Glaceon - Look at Leafeon. Gliscor - I may prefer Gligar but Gliscor looks more BA in my opinion. Mamoswine - Not used one but it does look interesting to use. Porygon-Z - Like it. Gallade - I personally prefer Gardevoir but I still like Gallade a lot. Probopass - One of me least favourite Pokemon (I like all Pokemon though). Dusknoir - Interesting. Froslass - Is equally awesome as Glalie (did I spell the name right?) Roserade - I don't mind it.
Bad: Electivire- It looks like a big, ugly... yeti? An electric yeti? Lickilicky- Take an already lame Pokemon, make it fat, give it an ugly curl, and give it the derpiest face ever. Probopass- It's like they were trying to make the worst design possible. Rhyperior- Needs no explanation. Rhydon did not need an evo, period.
Neutral: Ambipom- Don't really like Ambipom, but it's an improvement over Aipom, and it needed an evo. Gliscor- Pretty much the same as Ambipom. Magmortar- Don't really get the hate. Not my favorite, but it certainly looks cooler than Magmar. Magnezone- Eh. Magneton might be a lazy design, but Magnezone just looks kind of awkward. Mamoswine- Not really my favorite, but it's quite an improvement over Piloswine. Tangrowth- At least it's better than Tangela... Yanmega- It's okay. Can't say whether it's better or worse looking than Yanma, though it needed an evo.
Good: Dusknoir- Such an improvement over Dusclops. Froslass- To say she's an improvement over Snorunt and Glalie is a huge understatement. Gallade- Gardevoir is one of my favorites, but Gallade makes a perfect counterpart. Glaceon- Great design, like most Eeveelutions. I just wish it were available more easily. Honchkrow- Take Murkrow, make it a mob boss and give it a stylish hat. Leafeon- Finally, a Grass type Eeveelution, and another great design. Mismagius- Evo to my favorite Pokemon, and she looks cool. She's got a witch's hat and robes! Porygon-Z- I like the idea of a "corrupted" Porygon that was inflicted with a virus. Roserade- Roselia gets a cape and a cool hairdo. Togekiss- Togetic has always been rather underwhelming, but Togekiss was quite the improvement. Weavile- Sneasel was never my favorite, but Weavile made it look much cooler.