I liked the first one when it first came out and some of those actions scenes still hold up extremely well. But the sequels are just horrendous - especially the 4th one. But they're all bloated. And Bay just doesn't human very well - his jarring tonal shifts make the Star Wars prequels look good and his humor is dork vomit. Mainly, though, the guy just didn't understand giant robots and what the concept artists were offering him. A lot of those original designs were just so fucking cool - but not for a cartoony tone he was going in. They looked more like H.R. Giger Transformers. But he had them over lit, over saturated before asking animators to have them fight like 145lbs kung fu fighters - just contradicted everything, turned it all into a mush.
I do appreciate his love for beautiful, more superficial things, though. And his flicks have a unique sense of scope. I loved the score, too, and admired a lot of the behind the scenes technical stuff.
That said, the only Transformers I've ever liked were Beast Wars: