The below repost is based on the summary data released publicly by the FCAI and is a partial view of the market.

Overall
The market was down 9.3% with road bikes being the main contributor to this decline (-15.9%).
All road brands posted declines, with KTM (+6.2%), Husqvarna (+60.3% to 186 units), Indian (+20.2%) and Victory (+3.6%) the only brands in positive territory this year. 2017 was also the year Victory announced it was closing the brand so 2017 saw dealers run out of stock.
Yamaha had a particularly tough year in road bikes with sales down 28.4% (2,204 units). The 4 LAMS bikes reported (MT07L,YZF-R3A, MT03LA, XVS650) are responsible for 1,395 of these lost units. This saw Yamaha (road) market share drop to 13.8% from 16.3%
Prestige / Euro brands
Overall sales of the euro brands dropped in line with the market (-14.7% combined)
Ducati fared best only dropping 2.2% (to 1,968 units) while Triumph had a tough year dropping 26.4% to 2,301 units.
Aprilia and Moto Guzzi had a pretty terrible year as they transitioned to a new importer (PSI) who should hopefully have the foundations in place this year to turn around the fortunes of these brands.
Cruiser
Harley, the leading road bike, is losing its stranglehold on the top 10 Cruiser bikes with only 6 of the top 10 spots for the year.
After YOY growth for a number of years, Harley had a down year, dropping 1,379 units on 2016 (-13.4%).
The XG500 which has powered a lot of this growth in recent years, is now 3 years old, and as such, there are plenty of used models on the market now. Sales of the XG500 were down 29% (482 units) YOY
Victory announced it was closing the brand at the start of the year, so the run out of stock helped Victory post modest growth for the year (+3.6% to 284 units)
Indian also posted growth for the year (+20%). The Scout has been fantastic for Indian for the second year in a row it was responsible for 50% of India units sold (383 out of 769 units sold).
The future for the cruiser market looks bright with 4 LAMS bikes in the top 10 cruisers models sold (see below for more detail)
LAMS
The biggest segment of the market and the lifeblood for the future.
Road LAMs bikes appear to have taken a harder hit. While Road bikes were down 15.9% for the year the 20 road LAMS models (including the postie bike) that appear in the end of year report & Hyosung combined, were down 22.6%, well outstripping the broader Road category decline. (these 20 bikes represent 32% of the road market)
Cruiser LAMS bikes were more resistant to the downturn with the combined sales 4 LAMS cruisers (XG500, CMX500, Vulcan-S & XVS650/A) only down 5% (to 2,855 units)
In 2017 Hoysung, a brand that retailed almost 1,000 units in 2014 quietly left the market.
Scooters
The scooter market continued its long term decline with only Suzuki in the positive (+5.8% / +22 units to 404).
PSI group taking over the distribution of Aprilia will see their already dominant position grow stronger, based on all of 2017. While the transition of the distributor was happening, 2018 should see them easily own over 55% of the market.
Dirt bikes
Dirt bikes were down 5.9% to 37,379 units. The main mover and shaker in this category were Husqvarna, with 19.9% growth to end the year on 2,461 units sold (and 6.6% market share; up from 5.2% in 2016) , whiles it stablemate KTM was at the other end of the spectrum as the brand that took the biggest hit, down 17% to 5832 units sold.
Honda grew 1.6% (to 10,004 units) and grew its market share to 26.8%. All other brands posted declines.
Like LAMS bikes, kids bikes are the lifeblood of the dirt bike market and account for roughly 1/3 of retail sales. 6 fun bikes are in the top 10 dirt bikes sold and sales of these 6 declined slower than the wider dirt bike market at -3.4%, which bodes well for the future.
ATV
The ATV market was pretty flat selling 22,684 (-0.7%) units for the year (this number is incomplete as John Deere, Kioti, CF Moto and others do not supply sales data to the FCAI)
This was the first year with SXS broken out in its own category and it illustrated the changes happening in this market. Sales of the top ten ATVs were down 6% to 6438 units combined while sales of the top 10 SXS were up a whopping 58% to 5319 units
Polaris was the main mover in the market selling 9% more units that last year (6,583 units in 2017) and growing its market share from 26% to 29%. The majority of it sales are in the more profitable / expensive SXS market.
Kawasaki was the only other OEM in positive territory growing 3.2% (45 units). All other OEMs posted declines of between 3.5% and 6.1%.
The flat market is a surprising results considering farmers were investing in machinery this year. 2017 was a record year for tractor sales with sales up 8.1% coming on top of a very strong 2016.
The brand and category summary reports can be downloaded here