Although it was released in october, I just made the upgrade few weeks ago, and for my surprise, the new version (v1.5.0.1187) introduced some usability issues:

Startup time

It is incredibly slow to open. It takes at least 10 seconds to open, but sometimes it takes longer.

Changes in shortcut keys

The previous version used the number keys as shortcuts, like * – star, 9 – archive, 3 – mark unread, 8 – mark spam; and although the Nokia E61 has a full keyboard, you hadn't to hold the "blue" key (a kind of Shift-key) to activate the shortcut; you just pressed the corresponding "letter" key.

In the new version, I presume they tried to use the same shortcut keys used in normal GMail website: S – star, Y – archive, ! – spam.

Although it is a minor change, it is annoying when they change something you are used to.

Removal of shortcut key

The previous version had a shortcut key of "3" to mark a message unread. This is useful when you want to take a quick look at some big message and want to mark it unread, so you can read it later in your desktop machine.

In the new version they just removed this shortcut key! Please, don't remove features users are used to!

Introduction of not-so-shortcut keys

Some shortcut keys now requires you to press 2 keys simultaneously! For example, the shortcut to "search" is "/", but it doesn't respond if you just press the corresponding key for "/", because in Nokia E61 keyboard "/" is "Blue" + "I". To report spam, you have to press "blue" + "Q" (= "!"). They made it harder to report spam!

The previous version required only 1 key press! 

Conclusion

The previous version worked very well with the Nokia E61 keyboard and was very quick and simple. The new version is slower and commands are hard to activate due to the changes they made in shortcut keys.

These are serious issues (IMHO) because we are talking about a mobile application. In fact, I'm desperately trying to find some place to download the previous version. If you know where to download it, please post a comment.

PS: If someone from Google is reading this, please take it as a constructive criticism, and please, fix these issues