I've used Diskeeper for years, its outstanding, never had a single issue with it. It will even sort out Network Servers etc etc - superb piece of software.
One example of many: I run Climate Models in the background on my PC from www.climateprediction.net ((good thing to get into for anyone in that kind of thing - see the Site). The models back off processor use when other things are going on, so you end up utilising otherwise unused capacity. The models use hundreds of small files, and fragmentation can be an issue - but with Diskeeper running in the background as well, its no hassle. Diskeeper recognises the modeling going on, and will back away from that as needed. I never really know its there. Brilliant.
So I end up running the modelng, Diskeeper, playing a full blown GalCiv game, the odd few "cntrl-esc" sessions back to desktop during GalCiv games running browsers/mail etc, so processor (AMD FX60 Dual Core) is used absolutely to the max - and not a single contention or hassle. If Diskeeper works through that lot, as it does, you cant ask for better. Superb piece of software that is extremely well supported.
All the features on it are well thought through, all provide value, and there is no "Fluff" about them, they are there for a purpose that gives real benefit, not Fluff to mesmerise the unwary - often the case with freebie ones where they do a good job but you "dont know what you dont know" and the additional features from this paid for version are absolutely 100% worth it.
Regards
Zy