I am a tutor and I use the full Nessy learning programme a lot.
I like it and use it loads but I would say the following:
It is aimed at ages 7+, it can be used for children in Year 2, but Year 1 might be too young for it.
It does need to be used in a way that plans and integrates with the rest of the phonics teaching, You can't, in my opinion, just send a child off to "do Nessy". It could be managed/supervised by a TA but would need a teacher to decide/plan what parts an individual child should do as part of a structured phonics intervention.
Children love the games and earning nuggets to spend on the "fair" but the games could get boring if they are doing them too often. If they are doing them in every year, I think they would get fed-up with them.
The "fair" needs supervising so that student don't just waste time there. I only let my students go to the fair when they have earned 100 nuggets. In fact I think children could "waste" a fair amount of time not doing anything constructive, if they were left to do Nessy unsupervised.
The card games are great, but need time to make up. There are only a few different types of game though and again children can get bored if they are done too often.
The worksheets (in my opinion) tend to be a bit simple for some students and are not enough to cover a particular sound for a lot of students. I supplement them with a lot of other phonic material. I like the "speed reading" sheets though.
It does not cover all the graphemes children need to know.
In my opinion it has too much on certain graphemes and not enough on others.
Some of the stuff it does is not synthetic phonics ( ie learning the spelling of "tricky" words via mnemonics).
Some of the reading games can be done too easily, just by looking at the initial letter of the word rather than actually reading it.
I don't use the monitoring system to track where children are at. I keep separate notes and think this is better as the system will telly you they got 7 out of 10 in a game but won't tell you why (ie what mistakes they made or what they found hard).
I don't use the lesson plans provided by Nessy so can't tell you if they are good or not. My concern about them if followed rigidly is that they would only provide a couple of lessons for practising each grapheme and in my experience children learn these at different rates, many needing it to be covered in at least 5 lessons to really retain it.
I am concerned about you saying this: "for children where everything else has failed & they just arent getting it." You can't just get children to "do Nessy" and it will miraculously improve their reading. I find that it is great for a 10 minute slot in a 1-to-1 phonics lesson as a bit of a change and to provide a fun/interesting activity, and the cartoons can be a great way of introducing a grapheme as they are funny/memorable but I wouldn't use it as the total sum of my lesson. I'd suggest using something like Dancing Bears or Toe by Toe as the backbone and Nessy as more of a supplement. In my lessons, I tend to plan an individual order of working on graphemes (depending on the individual child and what they know already) and also do either Toe by Toe or Dancing Bears as I can leave this for student to do with a parent and it provides loads of great practice in blending etc. I then use a mixture of Nessy activities and others to provide practice in the particular phonic skills/knowledge I want that child to develop.