If memory serves correctly, the Glenfiddich Solera Reserve is a 15 year old malt.
It is generally acknowledged that the Glenfiddich 12 year old entry level malt is a bit simple and unchallenging for most malt drinkers, but the older Glenfiddichs offer much more in the way of flavour and complexity. So in theory, your 15yo should be a notch above "ordinary"!
I believe the Glenfiddich solera works in much the same way that a sherry solera system is operated. Instead of all the casks being combined for a single, once-off vatting which is subsequently bottled, the solera is a large vat which is continuously topped up with new malt each time a portion of it is drawn off for bottling. So at any one time, the solera has a portion of whisky that has recently been added, combined with portions that have been in the vat for considerably longer - arguably a portion from the very first original filling should still be present. So the solera is a constant work in progress, with the new and old constituents continually being married and altered with each subsequent top-up. "The Living Cask" by Loch Fyne Whiskies operates in much the same fashion.
I don't know whether the Glenfiddich Solera Reserve has more of a sherry influence than the regular 12yo, but many seem to think it is smoother, sweeter, and creamier than its younger stablemate. When I last had a bottle, about 18 months ago, I recall finding it a little out of balance, but I've read other posts on this forum from people who enjoyed it tremendously. I suspect your conclusion of "decent, but not great" is appropriate.
Hope this helpful.