RARO  Actually a great app indeed. I have used it extensively. But after a while, I wasn't able to access certain Mailboxes anymore. The only measure that helped was to disable MailTags.   
(Version 2.2.3)

EJI  Looks like it conflicts with WideMail v.51. The Mail.app message window won't open, and if you manually select it from the Window category in the menubar, it brings up one messy mail interface. Crap.  
(Version 2.2.2)

MALISTA  I've been using MailTags since day 1, looking for an alternative to Entourage which had better features at the time.

MailTags saved Mail for me, no less!

its free working sibling must be mentioned: Act-on.

bring QuickSilver in the equation, and your workflow becomes seemless.

highly recommended.  
(Version 2.2.1)

ONEBLESSEDGUY  I don't understand the review below. I have installed it and it works great. Perhaps there is an incompatibility with another plug-in that the reviewer has installed.

At any rate, I wanted to post something that states how much this program has already helped me and I've only been using it about 2 weeks! First, because I was using Mail Act-on, this program was easy to implement b/c I already had many rules set up. I was able to adjust my rules to include mail tags and get even better filtering and email flowthrough!

Second, b/c of mailtags I was able to implement a GTD workflow of my emails that has left my inbox empty and my to-dos organized. I actually enjoy it when I get an email b/c I get to organize it away. I'm ready to handle more than I feel like I ever have been.

Third, the hardest thing to do was to set up what I wanted in a workflow, did I want to use a Today, Tomorrow, Next Week, This Month kind of smart folder and file accordingly or did I want to use a color system with colors changing as the day approached. Almost anything is possible with MailTags.

Try it out! I don't know what I did with my emails before I did this. But I know I wasn't as efficient as I am now.  
(Version 2.2.1)

EGN  This is definitely not there yet. Just messed up the messages that I tagged. Could not drag them to any of my forlders; they just popped back in to the orginal folder. Not even after deinstalling. I had to delete the messages to get them off. Anything MT touched it messed up. All the other emails that I did not tag with it were OK. So it's off my Mac. This is the 2nd time I've tried to use it and the second time it does something I don't like. !  
(Version 2.2pb5)

BLLOYD  This mail plug-in has been fantastically useful for me. I'm not using all of the functionality, but rather just adding tags to messages (waiting, follow-up, etc.) and using the OmniFocus plug-in so I can quickly move to-do items as follow-ups over there where I manage them in a centralized location.

I tried to work with Leopard's to do functionality but it had some real limitations which made it unusable for me (specifically, if you have a few hundred to-do items over the course of all time with Leopard, which I did, it breaks down).

The developer has been fantastic in quickly resolving issues, and in adding much needed functionality. This is a core productivity application for me, along with OmniFocus and Circus Ponies Notebook I can efficiently keep track of whatever I need to, and follow up in a timely manner. Nothing falls through the cracks anymore.  
(Version 2.2b24)

BOBVB  Seems all of the 10.4 features I had used are back

BUT

I went into the Mail preferences MailTag panel and deleted a keyword I had defined and replace with another. All my mail with that keyword also disappeared from the .Mac IMAP account they were in. Fortunately they were local copied on my shut down laptop and I was able to recover them by quickly turning off the wifi and copying them to the hard disk.

Still a bit scary and emphasis on the 'beta' still.  
(Version 2.2b12)

TUISHIMI  Leopard 10.5.1, Mail 3.0, latest Beta of Mail Tags.

Doesn't seem to be working, no keyword searches (dynamic folders) work. Could be I missed that it was not working in the release notes.

Uninstalling for now.  
(Version 2.2b12)

LEESA  I bought and loved this app when I was using Tiger. Now that I'm using Leopard the only thing I purchased it for is gone: todos. I loved being able to sort my inbox by due dates as well as being able to rename the subject of those items. I mainly used it for sorting ebills.

The todos Leopard Mail offers are useless for me, I never wanted to have to have another window open to see my todo items. And until Apple offers a way to sort by todos in the inbox, I almost don't see the point.

Developers: please bring back this functionality in MailTags! I feel like I spent the money for nothing. :(   
(Version 2.2b12)

BEBDABHRHBRWEFG 3R§5Y5H4WTEDSFB  version 2 seems a lot slower than version 1,x - especially when performing actions on multiple emails.

also. with v2 - when i have a group of emails selected and then control+click to add a mailtags keyword, mailtags drops the selection afterwards. mailtags v1 kept the emails selected so that, after adding a keyword, i could then perform another action on that selection. please revert to the v1 way of doing things!

[have tried to mention this one the developer's discussion forum, but after attempting to register twice i've yet to receive the email telling me my account has been activated - perhaps the developer isn't that interested in what users think, after all]  
(Version 2.0.1)