At the end of the day, this just doesn't feel like a Negima sequel. Yea it's got Negi and crew in it (eventually), but Negima was a ecchi comedy with some action later in series. UQ Holder is an attempt at action... with nothing else.
The store feels like Touta was created just to be able to make a sequel without Negi being the main character. I feel like Ken's thought process went something like this, "I want to finish Negi's story, but I can't because of licensing issues. So I need to make another character that will encounter Negi and then finish his story." Which wouldn't necessarily be an issue, except Touta (the main character) doesn't feel like a main character which makes the whole story hard to get into. It ends up being just waiting for Negi to come back so the store would start again.
Without ecchi, Ken's art is actually just normal. It's not bad, but there isn't anything defining about it. It's just Meh
The character are by far the best part of this manga. Decent variety with some cool ability's. Although I do feel like the characters are all on the 'safe' side. Nothing offensive, nothing controversial, just some decent bland characters with cool ability's
When it comes to enjoyment the whole thing takes a nose dive. Putting Decent characters in a bad story with mediocre art isn't really an enjoyable experience. I forced myself to get to chapter 150 because somebody on the internet insisted that it got better, but it was a total waste of my time. I usually read manga at night before bed, and this just kept putting me to sleep.
Overall it's a 5, Mediocre. I really feel like that describes this manga. There are things that I feel are bad, but there are things I feel are good as well, and they balance each other out into a mediocre experience.