I haven't yet found the 'one-and-only' navigation app, and probably never will. Anyway, I quite like having more than one source of chart data as it can be a useful cross-check. I've been running Navionics+ on my B&G plotter for 6 years and I do like it, but as you say Graham, they are rather coy on their website about their pricing structure. Updates for the plotter version seem to be vastly more expensive than the phone/tablet versions. (Although they do let you view their charts online for free). However, it really can't be beaten for the detail of its bathymetric data, although UKHO comes close. If you like exploring up rivers and small inlets then check out the difference between Navionics and C-Map (or others) for the upper reaches of the River Dart, or the Avon at Bantham. Navionics is the only chart I know that will get you into Bantham. Garmin bought Navionics a while back so Navico, who own B&G, Lowrance etc, have bought C-Map so that they don't have to promote a competitor's product. I got a free copy with my B&G Vulcan but it was awful, so I paid for Navionics. I think they're steadily improving it but it's still not as good as Navionics.
My preferred alternative to Navionics is MemoryMap, which offers an annual subscription to all 850 odd UKHO charts for about £25 a year. I like it because:
I can also use OS maps on it when ashore.
One licence covers you for desktop PC, laptop, tablet and phones etc with syncing between them.
It's a doddle to plot a course and follow it, and it records your track.
It's got a simple toggle between the map and a data page, and the data display can be configured how you want it. A particularly neat trick is that you can have different data pages if you hold the phone vertically or horizontally.
I like the UKHO raster charts as you don't need to zoom in to check for hidden detail.
You can transfer charts onto devices for off-line use.
I've been using it for yonks so it's what I know
What's not so good about MM:
Transferring charts to devices is a bit of a game I'd prefer not to play.
It doesn't automatically change charts as you scroll or zoom, you have to select them manually
I also occasionally use Aquamap on my phone/iPad. £13.49 per year gets you the UK south coast, or I think about £20 gets you the whole UK. You can download chunks of your paid for area onto your phone as you need them. I like Aquamap's display which is very clear, and it also allows you to overlay satellite images with variable transparency, which is handy for checking out how well the chart lines up with reality. It also has a good anchor alarm which I use in preference to keeping my plotter switched on all night. (I usually forget to turn it off in the morning so get a fright just as I'm leaving the anchorage and the Klaxon goes off - happens every time!)
Just had a quick look at Savvy Navvy but can't say I was tempted by it. Interested to hear if anyone's got anything else they'd recommend.