I've seen this a few times now (a subtitled, widescreen print from Australia's sadly defunct SBS TV channel) and it never fails to disappoint.
I agree that the atmosphere is extremely spaghetti-esque - almost to the point of self-parody - and that there are good moments along the way, such as the shots through coloured panes of glass (I thought more should have been made of these), as well as a decent score, but overall the film barely has a pulse.
Alfonso Balcazar was much better as a writer/producer than a director, as this plodding revenge film underlines. It's slackly paced (others seem to have enjoyed the Russian roulette duels; I found they merely padded out the narrative) and has one of the most cringe-worthy flashbacks I've ever seen: a soft-focus reverie in which George Martin's character remembers his lost love, accompanied by images so trite they could have been ripped from the pages of a Mills & Boon romance.
I'm not dismissing it entirely, however - Gilbert Roland and Jack Elam are as solid as ever, with Elam dressed almost exactly like Frank in Once Upon a Time in the West (I'm not certain of the chronology of production, but Sonora was released in Italy before Leone's film). The veteran actors are a counterweight to George Martin's dull hero - he was far more effective as a villain than a leading man, in my opinion.