Although I've only had my Satmap for 2 months, so far, it's done everything I've required of it.
Given that I acquired mine with the full GB 1/50,000 mapping for a very similar, if not better, price than the Memorymap 3500, I would say that not only does the Satmap appear to be a far better device (based upon customer reviews I've read for the Memorymap 3500), it is also better value. Additionally, the Satmap (so far) has worked well and proved dependable. Admittedly I've only used it for short walks of a maximum of about 2 hours, but the rechargeable battery life appears to last well, and I've had no trouble picking up a satellite fix even beneath dense tree cover. I've also found it to give reasonably accurate grid references which I've used for plotting a survey in a cad program. One minor gripe is the awkward way of changing from the rechargeable battery to AAs, which could for example be a bit of a pain if the rechargeable pack runs out in the middle of a hike, say in lousy cold, wet weather somewhere out on the moors, and spare AA batteries are used instead.
The only couple of major downsides I've experiencesd (which are no fault of the device itself) is the expensive 1/25000 mapping, plus the fact that the map cards won't work on other gps devices, or a pc. Additional mapping software still needs to be purchased for plotting and / or displaying routes (on Ordnance Survey maps) on a pc, unless an annual subscription to the Satmap internet based mapping service is taken out, not good in my opinion.
Based on my (short) experience to date, I can thoroughly recommend the Satmap (especially if it comes bundled with the full GB mapping), but with the caveat of rather expensive additional mapping, for which I'll give the product 4 out of 5.