I have OnOne, NIK and Topaz and of the three I find I use Topaz most often.
You don't need the full suite, which can be quite expensive.
I find I use Adjust, Detail and Clarity the most, although not on every image and certainly not all three on the same image!
Denoise is also very good when you need it.
I was buying the programs one at a time, usually when they were on special offer (which is quite often).
After I had bought about 5 programs I went to buy another and I had a message that said I was eligible to buy the whole suite at a special price, so I coughed up, although I must admit there are some progs that I've never used seriously.
One thing from NIK that I use is some of the border effects which are not available in Topaz.
I hardly ever touch OnOne, since I can find everything I need in the the other two suites.
It's worth noting that Topaz frequently run Webinars to illustrate the uses of their programs.
If you sign up for one of those they always give away a time limited discount code (between 25% & 50%) at the end for the program they have been showcasing, so you can pick up a good deal that way.
Again, if you shop around you can usually find a discount code for NIK.
Nobody ever pays list price for any of these programs.
I don't use plug-ins on every image, and most of the time I find I can get all the adjustment I need in Lightroom alone, but Topaz Adjust has the ability to add a bit of "punch" to an image that I cannot always get in Lightroom alone.
at what stage in the work flow do you use noise reduction - beginning or middle or end ......... or at no particular stage?
I tend to use Denoise at the end of processing but there is no hard and fast rule, so I guess you could say "When you need it" although you must bear in mind that ANY denoising software tends to remove fine detail.