I've tried MarinerWrite because i've searched for the best text editing program on the mac (where "best" means the best for my needs, so this is no gerneral rating for everyone).
While the program worked fine and has some nice features (nice drag and drop, columns, mail merge, tables) and is easy to understand and to use, I notice that its base is out of date.
It feels like an OS 9 application and the port to OS X is not nice done. So it looks like the aqua-style buttons, pdf-support and font-smoothing are the only techniques that made it into MarinerWrite in the now nearly 5 years of OS X (not to mention the OS X beta-phase).
There's no unicode support, the toolbar is not customiseable like all other OS X programs, PDF as pictures don't work and the toolbar-icons are dithered 256 colors pictures. If you select some lines of text, it would look terrible because there are white "noise" between the letters and the blue-coloured selection area (compared to TextEdit, Pages, Mellel or NWE). Every dialouge-box pops up with the OS 9 zoom effect and the general userinterface is also not very well developed (editing and configuring tables, preferences)
From a typographer's point of view, there are all of the terrible faux-text-styles of the pre OS X area supported like compressed, extended, shadows and wrong italics (in OS X italics are only shown, if there is a italic version of the selected font installed, MarinerWrite and AppleWorks could make every font italic witch don't look nice). Also the manual is very bad laid-out, so if this application is on the road since the days of OS 8.6, there was a VERY long time, to improve such things, but it seems that nothing has happened (I hadn't tested any older version so there may be some (or some great) improvements but compared to pages, mellel and nisus writer express it seems, that MarinerWrite is some years behind in the terms mentioned above)
Also the table feature is not very impressive. While it works, it could happen, that if you add columns, the table grows so wide, that you couldn't see it entirely and so you couldn't resize it (only if you resize the whole table in the setup dialouge. Also the colour and pattern options to fill the table background with (dotted patterns that should look like semi transparent color areas instead of - for example - real transparency support) are one of the pre OS X indicators.
So to give a conclusion. It works as described and if the features it offers matches your needs, you could use and buy it. I couldn't find big problems or bugs so it get's 4 stars for working like they promise.
But because the developers didn't catch up with a full OS X support (opentype, transparencies, pdf drag and drop...) in nearly 5 years, I think this application will disappear from the stage within 2 years. If you look at the speed at which mellel is being developed (the feature-list of the next 1.9 and 2.0 is very promising) or take the new 1.0 version of Apple's Pages or the (maybe) upcoming Aqua-version of OpenOffice or look at AbiWrite, one or more of those applications will surpass MarinerWrite within a year in every respect and a year later no one needs MW anymore, if the developers won't make a really HUGE step forward (even quark gets unicode support this year with version 7 and they are really late).
I don't want to draw a black picture of MW but if there don't happen something, its days are numbered.
For now it works quite good but I have choosen mellel for my needs.
BTW: One poster mentioned that MW could open WordPerfect files. As i have tried, this is wrong (Mariner don't even say that it works). The only app on the Mac (OS X) that opens this files AFAIK is AbiWord (free and also nice but not ready for prime time).
Thanks for reading and it hopes the lines i dropped will help the customers _and_ the company to make the right decisions.

Mart°n