The pro-character builder argument: It is easy to use, prints nice character sheets, and generally updates quick enough that I can take advantage of it.
The anti-character builder argument is a bit harder to quantify, but we've been discussing it lately.
* When it is buggy, it is annoying. Particularly, it seems like bug fixes for certain problems have come slowly. I report the bugs as I find them, but I suspect that everyone who used to fix things is now working on the monster builder.
* As a social phenomena, the character builder provides a strong disincentive to actually coloring outside the box. Custom content is, at best, just a small note you get to provide. You can't specify things in enough detail that the program can actually use it. I've seen a few posts from 3rd parties saying that this contributes heavily to them leaving the market. It doesn't matter how good your custom class is if it is harder for me to use it than a WOTC class.
It actually goes beyond custom content. Some things are not well implemented. I'll give an example: Recently in my game, it was revealed to the players that the villain is trying to reassemble the Rod of Seven Parts. So I've been looking to see how artifacts are implemented in the CB. The answer is "not well". In WOTC's new artifact scheme, every artifact has 5 different stat blocks, depending on how happy the artifact is with you. The character builder only provides one of these stat blocks. So, once the player gets the artifact to an interesting power level, you get to do the work of maintaining it yourself. Joy! Some artifacts also offer special (and permanent) gifts after they leave you. But I see no way to actually add those to a character.
Anyway, it seems to me like if I want to add any custom content (or use certain pieces of official content), the character builder stops being useful, and becomes actively annoying. Maybe the 5e character builder will finally realize that people like to customize their games. A basic version of this in XML does not seem like it would be too hard to do.