I have a GSA1250 and a Tmax 500 Mk3.
I ride the Tmax daily 12 miles each way to work and back, plus shopping trips etc its a superb little bike and only costs me about £15 a week in petrol.
The Tmax is a simple enough machine to service and parts are pretty reasonable but you will need to invest in some special tooling (around £100) to renew the primary drive belt every 12,000 miles. The belts can be had from £70 for a Dayco to several hundred for an OE or a high performance Kevlar belt (such as those used in Tmax racing in Japan or Italy). I modified this tooling to also hold the clutch basket recently when replacing the clutch at 47,000 miles. It also needs fairly frequent oil changes at around 3500 miles as it has a fairly small oil capacity.
Due to its popularity in France and Italy there is good availability of new and used parts plus uprated stuff such as Malossi, Dr Pulley etc that can improve the pick-up and response such as new variator springs and weights, uprated clutches etc. Fowlers in Bristol do OE parts as do CMSNL in the Netherlands. Service kits can be had on eBay etc for sensible money, basically a full set of filters, two spark plugs and some oil. The biggest drama is the acres of interlocking body plastic that needs removal to gain access but you soon get used to it.
There is a good series of Youtube videos on Tmax mods and servicing from a guy called "49cc scoot" or something like that.
EDIT: try this for starters