It's got an absolute show-stealer of a villain. Varius Flavus is flat-out evil and a complete and utter bastard. He's an orgy-mad, greedy, thieving, physically gross murderer. His looting of the Roman Empire's tax receipts is so outrageous that even his fellow conspirator asks if he's overdoing it. When a tax inspector arrives to investigate, Flavus poisons him... and he has a poison ring on each finger and has run empty in the first two he tries.
"Ah, at last! I must remember to get refills."
In other words, he's done this before, almost certainly quite a lot.
He's so outrageous that we're cheering for his victim. Vexatius Sinusitus might be a Roman official and so technically our heroes' enemy, but he's honest, incorruptible and smart enough to stonewall his would-be murderers and send a message to a Gaulish druid (Getafix).
Flavus and Sinusitus are fantastic. When they're around, this is one of the strongest Asterix books. Unfortunately, there's a big middle section where Asterix and Obelix are jaunting off to Switzerland in search of a flower for an antidote. There's nothing wrong with any of that and I enjoyed it, but it clearly doesn't have the meat of its Flavus-Sinusitus bookends. It's basically just another light-hearted Asterix romp, with lots of jokes about Switzerland.
The Romans are orgy-mad. Goscinny and Uderzo were ripping off Federico Fellini's Satyricon, released the previous year. These "orgies" are family-friendly, obviously, but the book finds clever ways of making them seem depraved and yet also jolly and harmless for the children reading this book. (There's a cheese fondue penalty game that starts with beatings and ends with you being thrown in the lake with weights tied to your feet. Everyone thinks this is a hoot, including the victim, and on being pulled from the lake he happily returns to the orgy in search of more beatings.)
The jokes about Switzerland are fine. I felt sorry for a banker. There are also repeated references to Caesar destroying a bridge, which I guessed was perhaps another 1960s real-world gag... but no, it's historical. Caesar destroyed that bridge in 58 BC during a campaign against the Helvetii.
It's one of the better books in the series. The Swiss middle section is good, solid Asterix. Nothing special, but entertaining. Flavus and Sinusitus, though, are superlative and I can't believe there's anyone viler than Flavus in the rest of the series. Definitely recommended.