but like all plugins they aren't a be all and end all product, they do make the Photoshop experience easier in certain specific tasks. The Topaz interface is easy to use and in some of their plugins have a nice feature "I feel lucky" for when you're bored, it's a button with a pair of dice that randomizes the settings and can give you some unexpected results that lead you in a direction with an image that you might not have been thinking of originally.
That said, it's not a simple question to answer, I like Topaz but I also use the plugin bundles from Nik and OnOne and in the case of all bundles you will gravitate to some of the plugins in the bundle while others you will probably rarely use. Your best bet is to take advantage of Topaz's download and 30 day key and see for yourself.
http://www.topazlabs.com/downloads/index.php