I love this series and it's spin-offs Odyssey, The Helena Chronicles and Federation One.
The scripts are solid and continuity is well looked after. The production is so much better from say season 5 onwards - I'd say Episode 505 Epitaph is a good hopping on point as it begins the ongoing storyline that takes you right to the series finale, It's Hour Come Round At Last.
What I like about it most, and it is better watched as a full series, is that the characters they created were so well-developed over the course of 7 seasons and 50 episodes.
Not just Shelby, Lefler and the few (only a few) others from the TV Series, movies and at least one case novels, but their own Ro Nevin, Corey Aster, Jorian Zen, Henglaar, Myra Elbrey, Tolian Naros and a lot more were given full personalities and lives. Also a while before Phase II they presented the first gay storylines EVER on Trek - something Paramount/CBS has yet to do.
They have designed their own starship classes and upgraded familiar ones.
To me it was the 5th Series as when I started watching I shunned Enterprise! I'm a bit nicer to it now though.
As for the spin-offs even though they were divided into 3 series I count them now as Seasons 8-9-10 of Star Trek: Hidden Frontier as many of the same characters return, the storylines intertwine and are wrapped up in one massive joint finale 'Tossed Upon The Shore'. There are three Hidden Frontier [Excelsior crew] crossover movies [two shorts and a feature!] with Intrepid which I guess is unofficially [to me] a HF spin-off despite being independent and thousands of miles away. Characters from both have turned up as cameos in each other's shows, and both groups have shared resources, truly making it a fan film COMMUNITY which is wonderful.
I feel I cannot taking about Hidden Frontier without including Intrepid [which is still running] as it is so involved in that universe, it is also VERY well-produced, a bit different as in it has some physical sets and location shoots taking advantage of their beautiful local scenery. They use themes like Section 31 in their arcs and is set in a different part of space. It's nice to see them fly the British flag in the fan film community, again they have created a wonderful bank of characters who's story you want to follow episode-by-episode.
Simply put, Hidden Frontier and co to me are the definitive 'what happened next' in the Star Trek universe post-Nemesis. I miss them but like Admiral Chekov showing up in Renegades I'm sure a few faces will return someday