@whitey: is your switch IB or EN? Also, AFAIK, 2.4.x doesn't work with X2, only X3.
Just bringing some order to the table:
* MLNX drivers 1.8.x.x and 2.x.x - IB only (won't even work with EN cards). Both have IPoIB, but only 1.8.x.x have SRP and only 1.8.3 beta has iSER. 2.x.x can be used only for IPoIB networking, which means you can use it for iSCSI, but without RDMA. For IB, you always need an SM. These drivers support only X2 (only 1.8.x.x) and X3/Pro (both 1.8 and 2). Work only on ESXi 5.x and 6.0. No go on 6.5.
* MLNX drivers 1.9.x.x (from their site) - EN only. Will switch your VPI card to EN mode upon boot, hence expect orange lights if your switch is IB. Have iSER support. These drivers support only X3/Pro (not X2!). Work only on ESXi 5.x and 6.0. No go on 6.5.
* MLNX drivers 3.x (from their site or VMware) - EN only. Same as above, except no iSER. There is an inbox driver supporting X3 in EN mode on 6.5, but, again, no iSER
* MLNX drivers 4.x (from their site or VMware) - same as above, but X4 only.
Important notes:
* No IB support for anything above X3 (meaning no support at all for Connect-IB, because it's IB-only)
* No official support for anything below X3 (although unofficially 1.8.x.x work with X2 on 5.x and 6.0)
* No current SRP support (last working driver leaked 1.8.2.5)
* No current iSER support for anything except X3/Pro in EN mode (although there is 1.8.3 beta which works in IB mode for X2/3/Pro on 5.x and 6.0)
* No SRP or iSER at all on 6.5
Seriously, the situation around Mellanox support for ESXi makes me seriously think about switching to some form of KVM...