[embee]

I check mine every now and then, generally it's reasonably OK but does vary quite a lot depending on how/when you do it. I saw a guide (youtube maybe, i forget now) explaining a number of things to consider when doing it, things like resting, sitting, food etc. One of the things suggested was that the cuff (top of arm type) should NOT be too tight before running the test as this can alter the readings, which I have found to be the case. The cuff should be slack with just the loose taken up, definitely not pulled tight.
I was advised to get an Omron, as simple as possible (upgrades just get you more recording options etc generally, for straightforward checks an M2(+) does fine). Boots do their own branded version which is just an Omron M2 with a Boots label on it. When I got mine the Boots version was quite a bit cheaper than the Omron, but checking now it's the other way round, just see what you can find.
If you are of a certain age or disposition, blood pressure and blood sugar are very easy checks to do yourself regularly. Type 2 diabetes is in my family but so far I'm clear, I take after the side of the family with no real history so hopefully OK, I'm not overweight and have a reasonably healthy diet and lifestyle so just occasional checks seem fine. For blood sugar I used GlucoRx stuff which I've found easy to use and reliable, just bear in mind the advisory guidelines for use.