I have a Speedstroke and love it. It's an integral part of my training.
There's a very good review of the KayakPro Speedstroke Gym (SS) on this site, by Erik Borgnes. BTW, Erik leads a winter training group for paddlers who have a SS, with a weekly training program.
The SS is expensive -- basically the same as buying another kayak or ski. It took me a long time to bite the bullet and get one. I wanted a quality erg ever since I first used a (Dansprint) erg while in Sweden several years ago. I now consider the SS one of my all-time best purchases and highly recommend it.
Storage was initially a concern since the erg is long (necessary to replicate a smooth catch) and doesn't fold, but it slides up nicely against a wall. I keep it behind my coach on felt furniture pads and can quickly slide it into place for use.
The main appeal of the erg to me is that I can maximize my limited training time. Even with only an hour or so to spare, I can get a quality training session. Normally, just loading my kayak and traveling to and from the water takes 40 minutes or more.
It's warm enough where I live to paddle year-round, but work often makes it difficult for me to train on the water during the week. With the SS I can easily train multiple times per week.
While you don't get balance training on the erg (unless you make a tiling seat), the SS is very useful for work on technique and fitness. The action is smooth and feels like being on the water.
A long session on an erg can be boring, compared to being on the water, but the SS allows you to hookup the erg to a PC and large screen, where you can select multiple venues with very good graphics and race against live racers, or computer generated "rabbits". The software allows you to set these rabbits to your past-performances, or a set pace, so if you are competitive at all, you will not be bored. The software is not included in the purchase price and is not cheap, but I rarely use my SS without it.
The unit is well built and I haven't had any trouble getting questions answered by Gray or Steve. FWIW, I have no affiliation with KayakPro.
Greg Stamer