Both Bowflex and Soloflex are based on the principles of resistance bands with Soloflex actually using resistance bands on steroids to provide the resistance. Like all other resistance (weight) machines they restrict your range of motion and force you to move in a preset arc. The are needlessly expensive as are many of the other high touted and advertised exercise programmes and equipment. Both machines show up on the used market, I recently saw a Bowflex in a thrift store. The have frequent flier miles on Craigslist and other second hand fitness equipment stores such as Second Wind.
You actually need neither for a good strength workout, Lifeline USA sells some resistance band based home workout systems which are excellent alternatives. www.Lifeline USA.com
. Another alternative is suspension training such as TRX but there are cheaper alternatives to the TRX equipment which are as good and do the same thing. Investing in kettlebell classes to learn proper technique then investing in the proper weight kettlebells is much cheaper than a Bowflex or Soloflex.
I have experience with Soloflex, I own one and have had one off and on since 1984 but mine is currently up for sale since I have alternatives which I use and prefer.