GPS 

It does excellent job, often in a different way to what I expected. What we use it for -
 

• to see track on its little screen. It gives perfect idea of influence of drift and tidal stream. I normally set direction on Autohelm 800 and check on GPS if true direction is what I want. If not, small adjustment on Autohelm a Alka goes where I want her.
• 315 shows with marker where I'll be in 10 minutes if I continue with the same speed and bearing. If marker intersects waypoint, or buoy from CD, I know that it will be a close call even if bow of Alka aims in quite a different way. It is uncannily accurate.
• if you go through strange channel you can retract your trip - or section of it - quite accurately. That is often useful.
• when I leave strange harbour, I make waypoint mid of entrance. I keep setting this "home point" as "goto waypoint" and 315 will continuously tell me the distance. I have returned this way in the dark and my only problem was to believe what GPS is saying. It is accurate enough.
• 315 also tells me speed over ground, which is often handy and always interesting.
• when using paper map I set course and continue till lon or lat is what it should be (e.g. going "down" until I am "level with jetty", then turn East and keep going until I am close enough to see clearly). Obviously both values can be used at the same time. You can mark which is "safe strip" and keep boat inside of it. Easy and requires next to no preparation.
• 315 will tell me overall distance sailed - frankly next to useless, but VERY interesting. Ends in my log after every trip.
• 315 can provide NMEA signal to other equipment, incl. PC with scanned map. Problem is, that laptop is a bit costly thing to keep on board and most laptop screens are almost useless in sunshine, even in the cabin. I have tried this but did not fall in love with it so far.
• 315 logs track as you go and can download to PC later at home. You can superimpose it on a scanned map and within a few second you have exact track of your trip, including all mistakes, course adjustments and hesitations. It can go to HDD archive as TIF file or on printer.
• 315 has got anchor watch with many options.
• GPS can check car speedo - it is more accurate and possibly even more legible. If I set cruise control by GPS I tend to overtake most other cars - most speedos show more than what you make.
• 315 is mounted in cockpit on 2 velcro dots (second couple of dots is stuck on the other side of cockpit if I sit on the other side) 

All this plus number of other less important things make GPS tool which I use almost constantly for one or the other purpose.

If I would be looking for new GPS I would require as a minimum graphical screen and NMEA output to work with PC. Rest is interesting but not essential to us.
 

Notes: 

• map is not "registered" perfectly, it is obvious from section in Hastings channel (and at Tankerton jetty) that red track  is slightly shifted. I should rectify it, but it doesn't matter really - all I need to know is how much it is shifted.
• "spikes" in one section are most likely short timed inaccuracies in satellite reception which logging loop recorded as "important events".

I sailed back; return track being N of outgoing one at first. I just made sure that screen marker is always left of navigational mark at S tip of Middle Spit. At later stage I was simply aiming for my waypoint at entry to Hastings channel. 

I "cut corner" at Hastings channel as it was high tide now and I was able to get over shallows