The 540 is a pretty good strobe. I haven't experienced the problem with the Zoom lockup. Lots of features and lots of adaptability when you use it the with F5P cord and hotshoe adapter for off camera lighting, which gives you all the same options (P-TTL, AUTO, MANUAL) as if it were on the camera. Cool.
The only complaint I have of it is that the ring that you turn on the hotshoe to secure it can get a little loose over time. A number of users have reported tightening the loose ring so much that they actually broke it and the flash was stuck on the body.
The Metz DOES look interesting though. If I wouldn't have gone with the 540, the Metz 48, which is about the same price I would've gone with. The Metz 58 is rather pricey though.