Some people dislike the early model's heavier trigger pull - you can send it to Kel-Tec to have it replaced with a lighter pull. Some people claim that the original plastic guide rod is too weak and prone to failure - but I've never seen any actual confirmed case of failure (and Kel-Tec claims they haven't, either.) They do make a replacement steel rod, which they will do for you, too.
Their service is great, and very reasonably priced. I sent my 10+ year old one in for a refurbishing last year, and a full refurbishment, with all the 'upgrades' and trimmings, plus a few accessories (12 round mag, mag extension, wider trigger finger pad,) came to less than $80. And that was off-warranty, as I was not the original purchaser.